Move spring.oauth2.* to security.oauth2.*

Unfortunately, we have no other choice to flip the ignoreUnknownFields
attribute of `SecurityProperties` has many different target are now set
for that namespace outside the class. See gh-3445 for a potential way
to improve that.

Closes gh-3327

spring-boot-cli/src/test/java/org/springframework/boot/cli/SampleIntegrationTests.java

@@ -81,9 +81,9 @@ public class SampleIntegrationTests {
 	public void oauth2Sample() throws Exception {
 		String output = this.cli.run("oauth2.groovy");
 		assertTrue("Wrong output: " + output,
-				output.contains("spring.oauth2.client.clientId"));
+				output.contains("security.oauth2.client.clientId"));
 		assertTrue("Wrong output: " + output,
-				output.contains("spring.oauth2.client.secret ="));
+				output.contains("security.oauth2.client.secret ="));
 	}
 
 	@Test