'groups/mine' should return only the user's groups.

This fix reverts the accidental change made in the group
creation patch (334b86e7d6c196544413885d4abe798ffe1279b4)
and the fix needed to make it working correctly.

Now, 'groups/mine' will return only the groups that the user
is a member of, and not *all* groups.

Change-Id: I270f63ab9f06fc6fce34c355e8501cdd8345f14e