spawn cbq-engine if ENABLE_QUERY environment variable is defined

This is initial and simplistic integration of query engine into
couchbase. It does nothing else but spawns cbq-engine on every
node. cluster_run case is taken care of as well.

Change-Id: I9ae49132e86859986e2f813f0906374159b9e078
Reviewed-on: http://review.couchbase.org/41573
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>