MB-8189 Start ns_server application as permanent.

application:start defaults to temporary restart type. Which means that
if ns_server application terminates for whatever reason (say
reached_max_restart_intensity), the VM will still survive this and
babysitter won't know that something went wrong.

Change-Id: I59238ab4e332c738a82a70a91b2f4712c2b429d2
Reviewed-on: http://review.couchbase.org/26087
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>