Support Maven staged repositories

When WildFly Core is released, one of the step is to check that the Core releases can be downloaded from Nexus repositories to build WildFly.

However that step requires to release WildFly Core artifacts. If something went wrong, the release is out and there is nothing we can do except moving to the next release and restart the whole process.

We should instead ensure that WildFly can access Nexus staging repositories so that we can verify that WildFly can download and uses the Core artifcats before they are actually released.
If something goes wrong, we can redo the tag without any issue.

This simplifies the release of WildFly core:
1. the Core release is created as usual.
2. WildFly consumes the artifacts from the staging repository
3. If everything is fine, the last steps for Core releases are the Git tag and the actual release from the staging repository.