MB-10409: cluster_run changes to pass IPv6 param.
MB-10409: cluster_run changes to pass IPv6 param.

The cluster_run script has been modified to pass in the right
parameters to the "erl" command based on an environment variable
called "IPV6". The following is how one could start a cluster in
IPV6 mode:

"IPV6=true ./cluster_run --nodes 3"

Change-Id: Ib04b0c73c30e04191148b4eed9f5d2e451e05a48
Reviewed-on: http://review.couchbase.org/71875
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server