allow passing environment variables to misc:run_external_tool

Change-Id: I0e57f6b0018aac81912986722d550170e7d5e6b9
Reviewed-on: http://review.couchbase.org/33229
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>