[CONTINUUM-2724] Can't add a project over XMLRPC without a project group

Use a different permission check for this case, since it's about whether a
group can be added, not a project in the given group.


git-svn-id: https://svn.apache.org/repos/asf/continuum/trunk@1588418 13f79535-47bb-0310-9956-ffa450edef68