allow mavenLocal-path to be modified via System-property

allow the path mvnLocal() points to, to be modified via the 'maven.repo.local' System property as it can be done with Maven itself
add integration-test for mavenLocal-path to be modified via System-property
Merge pull request #315 from ChristophGr/maven_local_system_property

allow mavenLocal-path to be modified via System-property