added /diag option to do per-node-diag on one node

So that we can run them on many nodes concurrently.

Change-Id: I760864b35f59596a51e1dd206364cc6d2bde24d4
Reviewed-on: http://review.couchbase.org/37283
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>