Add easy mechanism to return backup volfile servers

[prashanthpai]

The backup servers are slightly less relevant as the glusterfs client (fuse,libgfapi) will get this list in the response to GETSPEC that happens during mount. Refer: gluster/glusterd2#382

However, clients will need failover during initial mount. More discussion here: https://github.com/gluster/glusterfs/issues/351#issuecomment-387633201

Some first thoughts on this mechanism:

• Implement it as an API in restclient but not as REST endpoint. The resclient can gather the backup volfile servers using peer list and/or volume list.
• Return this list as response header in volume status or volume start API.

[atinmu]

Not a blocker for GCS/1.0 based on the revised MVP.