Support new jbossBase property

Adding support of 'jbossBase' property in jboss-as-maven-plugin configuration (beside 'jbossHome') enables to distinguish a JBoss "home" reference installation (pointed by 'jbossHome') from a working separated installation (pointed by 'jbossBase', assumed compliant with standard JBoss directory structure, typically localized inside project), keeping JBoss "home" immutable (no deployment, no logging, no changes in standalone.xml once deployment is done ...)

Such architecture could be obtained by setting property '-Djboss.server.base.dir=${jboss.base.dir}/standalone' in <jvmArgs /> but it faces to a logging issue : bootstrap logging starts on JBoss "home" side then continues on JBoss "base" side.

The current patch simplify the configuration (no need of '-Djboss.server.base.dir' property in <jvmArgs />) and avoid the logging issue (see StandaloneServer.java refactoring)

Below an example of usage : 

		        <plugin>
				<groupId>org.jboss.as.plugins</groupId>
				<artifactId>jboss-as-maven-plugin</artifactId>
				<version>${version.jboss-as-maven-plugin}</version>
				<configuration>
					<jbossHome>${jboss.home.dir}</jbossHome>
					<jbossBase>${jboss.base.dir}</jbossBase>
					<modules-path>
						<paths>
							<path>${jboss.base.dir}/modules</path>
							<path>${jboss.home.dir}/modules</path>
						</paths>
					</modules-path>
				</configuration>
			</plugin>				

Note : user already is able to manage its own set of modules which can complete native set of modules from JBoss "home" (thanks to a recent contribution from Juan-Manuel Cabrera to 7.6.Final). 
If <modules-path /> is not configured while <jbossBase /> is, a next step would be to force the default of <modules-path /> like above...