Allow to declare setup projects which must be run before all the rest

For the ITs of the maven-release-plugin I needed a stub SCM provider. To make sure its artifact is available for all the IT projects, it needs to be installed into the local repo first. Since the build order of the selected projects is in general undefined, this requires to configure two plugin executions with appropriate includes/excludes. To avoid this hassle, their should be an easy way to have some projects be guaranteed to be run ahead of the other builds.