Use dedicated rolebinding to store owner

The following discussion from !4 (merged) should be addressed:

• @alossent started a discussion:

  Site owner may add other admins to the default admin rolebinding (e.g. by giving other users the admin role in the web console). So we cannot just replace the admin rolebinding on owner change, this will remove all other admins as well. I think the logic in current openshift-site-manager code is to remove the previous owner and add the new owner.

  A better solution could be to use another, dedicated rolebinding resource to store the owner and give it the admin role, which we clearly mark as being managed by us (set an appropriate description annotation). Then it's OK to overwrite any user changes. Maybe rolebinding/paas-site-owner?

  This means we leave the default admin rolebinding untouched. (I think it's created automatically)