MB-8656: allow adding vm flags of child vm via env variable

By setting environment variable COUCHBASE_NS_SERVER_VM_EXTRA_ARGS it
is now possible to add erlang vm flags of child vm. It is interpreted
as erlang term which must represent list of strings.

E.g. in order to pass +swt low you can do the following:

COUCHBASE_NS_SERVER_VM_EXTRA_ARGS='["+swt", "low"]' ./cluster_run

Change-Id: I172c2985dbbccdd139bb2ea929244d6ec0d8e866
Reviewed-on: http://review.couchbase.org/27578
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>