Add an RPC service to glusterd by which the cli can ask it to perform mounts on its behalf.

The mount are given access to only the user requested by the cli. Moreover, that user gets full control over the mount (allowed to do privileged operations). Granting mounts to users are controlled by glusterd volume config.

This way we can lease privileges over a volume to an unprivileged system user in a controlled way.

CHANGE: http://review.gluster.com/128 (Mountbroker is configured in glusterd volfile through a DSL) merged in master by Vijay Bellur (vijay)

CHANGE: http://review.gluster.com/432 (Change-Id: I8a88d2b9228c3614ee7cbaf48782a419e6aee0f6) merged in master by Vijay Bellur (vijay)

CHANGE: http://review.gluster.com/2817 (glusterd/mountbroker: fix handling of MEET mountbroker keyword) merged in master by Vijay Bellur (vijay)

CHANGE: http://review.gluster.com/2818 (glusterd/mountbroker: enhance mountbroker-geo-replication.* option to take multiple volumes) merged in master by Vijay Bellur (vijay)

CHANGE: http://review.gluster.com/3171 (glusterd / mountbroker: warn if mountbroker-root permissions are suspected to be too strict) merged in master by Anand Avati (avati)