make single_bucket_sup a parameter in ns_bucket_worker_sup

so the same supervisors can be reused for couchdb node

Change-Id: I8a753e5ec8cee5cfab6c581ba710e7d80d718b02
Reviewed-on: http://review.couchbase.org/41928
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
do not use couch_config from ns_config_default

since there will be no couch_config on ns_server node, use env variable
with default value instead

Change-Id: If4bc21d0f4bc2fef55be83e36aa7e70587a0cff8
Reviewed-on: http://review.couchbase.org/41930
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>