Add support for Azure authentication.

Closes gh-250.

docs/src/main/asciidoc/spring-cloud-vault.adoc

@@ -369,6 +369,50 @@ as the authentication implementation uses AWS SDK types for credentials and requ
 
 See also: https://www.vaultproject.io/docs/auth/aws.html[Vault Documentation: Using the aws auth backend]
 
+[[vault.config.authentication.azuremsi]]
+=== Azure MSI authentication
+
+The https://www.vaultproject.io/docs/auth/azure.html[azure]
+auth backend provides a secure introduction mechanism
+for Azure VM instances, allowing automated retrieval of a Vault
+token. Unlike most Vault authentication backends, this backend
+does not require first-deploying, or provisioning security-sensitive
+credentials (tokens, username/password, client certificates, etc.).
+Instead, it treats Azure as a Trusted Third Party and uses the
+managed service identity and instance metadata information that can be
+bound to a VM instance.
+
+.bootstrap.yml with required Azure Authentication properties
+====
+[source,yaml]
+----
+spring.cloud.vault:
+    authentication: AZURE_MSI
+    azure-msi:
+        role: my-dev-role
+----
+====
+
+.bootstrap.yml with all Azure Authentication properties
+====
+[source,yaml]
+----
+spring.cloud.vault:
+    authentication: AZURE_MSI
+    azure-msi:
+        role: my-dev-role
+        azure-path: aws
+----
+====
+
+* `role` sets the name of the role against which the login is being attempted.
+* `azure-path` sets the path of the Azure mount to use
+
+Azure MSI authentication fetches environmental details about the virtual machine 
+(subscription Id, resource group, VM name) from the instance metadata service.
+
+See also: https://www.vaultproject.io/docs/auth/azure.html[Vault Documentation: Using the azure auth backend]
+
 [[vault.config.authentication.clientcert]]
 === TLS certificate authentication
 

spring-cloud-vault-config/src/main/java/org/springframework/cloud/vault/config/ClientAuthenticationFactory.java

@@ -26,6 +26,7 @@ import lombok.RequiredArgsConstructor;
 import org.springframework.beans.BeanUtils;
 import org.springframework.cloud.vault.config.VaultProperties.AppRoleProperties;
 import org.springframework.cloud.vault.config.VaultProperties.AwsIamProperties;
+import org.springframework.cloud.vault.config.VaultProperties.AzureMsiProperties;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
@@ -80,6 +81,9 @@ class ClientAuthenticationFactory {
 		case AWS_IAM:
 			return awsIamAuthentication(vaultProperties);
 
+		case AZURE_MSI:
+			return azureMsiAuthentication(vaultProperties);
+
 		case CUBBYHOLE:
 			return cubbyholeAuthentication();
 
@@ -250,6 +254,19 @@ class ClientAuthenticationFactory {
 		return new AwsIamAuthentication(options, restOperations);
 	}
 
+	private ClientAuthentication azureMsiAuthentication(VaultProperties vaultProperties) {
+
+		AzureMsiProperties azureMsi = vaultProperties.getAzureMsi();
+
+		Assert.hasText(azureMsi.getRole(),
+				"Azure role (spring.cloud.vault.azure-msi.role) must not be empty");
+
+		AzureMsiAuthenticationOptions options = AzureMsiAuthenticationOptions.builder()
+				.role(azureMsi.getRole()).build();
+
+		return new AzureMsiAuthentication(options, restOperations);
+	}
+
 	private ClientAuthentication cubbyholeAuthentication() {
 
 		Assert.hasText(vaultProperties.getToken(),

spring-cloud-vault-config/src/main/java/org/springframework/cloud/vault/config/VaultProperties.java

@@ -98,6 +98,8 @@ public class VaultProperties implements EnvironmentAware {
 
 	private AwsIamProperties awsIam = new AwsIamProperties();
 
+	private AzureMsiProperties azureMsi = new AzureMsiProperties();
+
 	private KubernetesProperties kubernetes = new KubernetesProperties();
 
 	private Ssl ssl = new Ssl();
@@ -248,6 +250,21 @@ public class VaultProperties implements EnvironmentAware {
 		private String serverName;
 	}
 
+	@Data
+	public static class AzureMsiProperties {
+
+		/**
+		 * Mount path of the Azure MSI authentication backend.
+		 */
+		@NotEmpty
+		private String azurePath = "azure";
+
+		/**
+		 * Name of the role.
+		 */
+		private String role = "";
+	}
+
 	@Data
 	public static class KubernetesProperties {
 
@@ -328,6 +345,6 @@ public class VaultProperties implements EnvironmentAware {
 	}
 
 	public enum AuthenticationMethod {
-		TOKEN, APPID, APPROLE, AWS_EC2, AWS_IAM, CERT, CUBBYHOLE, KUBERNETES
+		TOKEN, APPID, APPROLE, AWS_EC2, AWS_IAM, AZURE_MSI, CERT, CUBBYHOLE, KUBERNETES
 	}
 }