glusterd: Authenticate management handshake requests

Management handshake requests, which are used to validate op-version
supported by the peers, are now only allowed if,
- the glusterd doesn't have any other peer, or
- the request was sent by another peer.

This prevents the op-version of a peer being changed because of a
connection attempt by an invalid peer.

Change-Id: I248c386ed5ec4f8360e7b5e7f9ab74b7e8a7fc65
BUG: 1109741
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/8126
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>