#SB-120: Assign ports to the maven-jetty-plugin, derby-maven-plugin and unboundid-maven-plugin by reserving free ones with the build-helper-maven-plugin

- Added a convenience method for getting a pre-configured instance of the ArtifactClient.
- NPE check/fix for the AssignedPorts.
Merge remote-tracking branch 'origin/master'

Conflicts:
	strongbox-testing/strongbox-testing-core/src/main/java/org/carlspring/strongbox/testing/AssignedPorts.java