Separate release mgmt permissions from review.

We don't want everyone in libra-core to be able to manage
branches and tags. This power has been moved to a new
libra-drivers LP group, similar to other projects.

Change-Id: I24557891bd0a61c57795e385f75aac43c54e448c
Reviewed-on: https://review.openstack.org/23816
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins