Make use of Gradle's application plugin optional when using Boot plugin

Previously, the Spring Boot Gradle plugin would always apply the
application plugin to a project. It then piggy-backed on the application
plugin’s mainClassName and applicationDefaultJvmArgs properties for the
configuration of the bootRun task.

This commit updates the Spring Boot Gradle plugin so that it no longer
applies the application plugin. If the user applies the application
plugin then its configuration will be used, but it’s a no longer
requirement.

Users who do not need the application plugin, but who were using the
mainClassName or applicationDefaultJvmArgs properties will need to
change their builds as a result of this change as those properties will
no longer exist. As before, the mainClassName can be configured on the
springBoot extension:

springBoot {
	mainClassName 'com.example.YourApplication'
}

The applicationDefaultJvmArgs property can be used, but it must now be
declared with the project's ext block. For example:

ext {
	applicationDefaultJvmArgs = [ '-Dcom.example.property=true' ]
}

Closes gh-2679

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -768,7 +768,7 @@ entropy for session keys.)
 
 [[using-boot-running-with-the-gradle-plugin]]
 === Using the Gradle plugin
-The Spring Boot Gradle plugin also includes a `run` goal which can be used to run
+The Spring Boot Gradle plugin also includes a `bootRun` task which can be used to run
 your application in an exploded form. The `bootRun` task is added whenever you import
 the `spring-boot-plugin`