REVIEW [NX-486] remove nexus-test-environment-maven-plugin

This is not a direct replacement of the functionality that existed in the `nexus-test-environment-maven-plugin`, but is enough to keep the legacy tests up and running until we can increase coverage of the modern tests (and means one less maven plugin to maintain).

If anyone feels strongly that the `nexus-test-environment-maven-plugin` should be kept, please speak now :)

https://issues.sonatype.org/browse/NX-486
http://bamboo.s/browse/NX-OSSF135-2