don't crash ns_node_disco_conf_events when ns_config_rep is down
don't crash ns_node_disco_conf_events when ns_config_rep is down

We had this catch before and it makes sense to keep it. If
ns_config_rep is crashed, it'll resync config. Which means that we can
ignore it's absense here.

Change-Id: I8b4cc8b6cd1a5d502a1cb671b9192289a26b9c55
Reviewed-on: http://review.membase.org/6250
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

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