WFBUILD-10 Add ability to override a specific version of an artifact from a feature pack

Artifacts can be overriden by setting:

<system-property-version-overrides>true</system-property-version-overrides>

in the maven plugin config. Versions can then be overriden using a system
property of the form version.G:A=V, for example:

-Dversion.io.undertow:undertow-core=new-version