ctdb-scripts: Add service_stats_command variable to NFS checks

ctdb-scripts: Add service_stats_command variable to NFS checks

When monitoring an RPC service, the rpcinfo command might time out
even though the service is making progress.  In this case, it is just
slow, so counting the timeout as a failure and potentially restarting
the service will not help.  The problem is determining if a service is
making progress.

Add a new NFS checks service_stats_command.  This command is intended
to run a statistics command.  The output is naively compared using
cmp(1).  If the output changes then rpcinfo failures are converted to
successes.

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>