Allow deployment to Docker group repositories

For deployments, it would be handy to configure Nexus group repositories to allow deployment requests by having a Nexus administrator set a default deployment repository. The deployment repository is expected to be a member of the group repo.

In this way, deployment tools and builds will only need to know about a single group repository URL instead of two URLs, one for push and one for pull.

This is also one less item to configure in a client tool and the control over repository deployment policy is put more directly in the hands of the repository manager administrator.