Fix spring.application.exclude YAML property

`spring.application.exclude` is retrieved via the
`RelaxedPropertyResolver` API explicitly and it does not have any
standard API to retrieve a list of values. As a consequence that property
could only be specified as a comma-separated value.

This felt convoluted in YAML. `RelaxedPropertyResolver` has now a
`getProperties` method that works with both comma-separated value and
index elements (i.e. list).

Closes gh-4352

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/EnableAutoConfigurationImportSelectorTests.java

@@ -134,6 +134,21 @@ public class EnableAutoConfigurationImportSelectorTests {
 						VelocityAutoConfiguration.class.getName()));
 	}
 
+	@Test
+	public void severalPropertyYamlExclusionsAreApplied() {
+		configureExclusions(new String[0], new String[0], new String[0]);
+		this.environment.setProperty("spring.autoconfigure.exclude[0]",
+				FreeMarkerAutoConfiguration.class.getName());
+		this.environment.setProperty("spring.autoconfigure.exclude[1]",
+				VelocityAutoConfiguration.class.getName());
+		String[] imports = this.importSelector.selectImports(this.annotationMetadata);
+		assertThat(imports.length,
+				is(equalTo(getAutoConfigurationClassNames().size() - 2)));
+		assertThat(ConditionEvaluationReport.get(this.beanFactory).getExclusions(),
+				containsInAnyOrder(FreeMarkerAutoConfiguration.class.getName(),
+						VelocityAutoConfiguration.class.getName()));
+	}
+
 	@Test
 	public void combinedExclusionsAreApplied() {
 		configureExclusions(new String[] { VelocityAutoConfiguration.class.getName() },