Support 'required properties' precondition

Users may now call #setRequiredProperties(String...) against the
Environment (via its ConfigurablePropertyResolver interface) in order
to indicate which properties must be present.

Environment#validateRequiredProperties() is invoked by
AbstractApplicationContext during the refresh() lifecycle to perform
the actual check and a MissingRequiredPropertiesException is thrown
if the precondition is not satisfied.

Issue: SPR-8323

org.springframework.context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java

@@ -488,6 +488,10 @@ public abstract class AbstractApplicationContext extends DefaultResourceLoader
 
 		// Initialize any placeholder property sources in the context environment
 		initPropertySources();
+
+		// Validate that all properties marked as required are resolvable
+		// see ConfigurablePropertyResolver#setRequiredProperties
+		this.environment.validateRequiredProperties();
 	}
 
 	/**