maven plugin is moved off to a separate repository.

See https://github.com/jenkinsci/maven-plugin

(cherry picked from commit f98070d3)

Conflicts:
	maven-plugin/pom.xml
	maven-plugin/src/main/java/hudson/maven/AbstractMavenProcessFactory.java
	maven-plugin/src/main/java/hudson/maven/MavenModuleSetBuild.java
	maven-plugin/src/main/java/hudson/maven/MavenProcessFactory.java
	maven-plugin/src/main/resources/hudson/maven/MavenModuleSet/configure-entries.jelly
	plugins/pom.xml
	pom.xml