protocol/server: Validate outstanding-rpc-limit

server.outstanding-rpc-limit by passes validation enforcement of gluster
as protocol/server ignores any keys starting with rpc (rpc* wildcard).
Which also causes a side effect, i.e. help text gets displayed as NULL.

Change-Id: I925e028529d671e1e29a0c7ff7e625fdb49a1be4
BUG: 1091364
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/7562
Reviewed-by: Poornima G <pgurusid@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>