supervise ns_config_events subscription in ns_bucket_sup. MB-4400
supervise ns_config_events subscription in ns_bucket_sup. MB-4400

So that restarts of ns_config_events for whatever reason does not
impact our ability to create/delete buckets.

Change-Id: I44a19053e72e35d9ec8d0cdc1db21901640fe134
Reviewed-on: http://review.couchbase.org/10636
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,vzasade/ns_server,vzasade/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,daverigby/ns_server,daverigby/ns_server,membase/ns_server,ceejatec/ns_server,daverigby/ns_server,membase/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,ceejatec/ns_server,vzasade/ns_server,membase/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server