bp: return 503 when trying to create bucket during rebalance
bp: return 503 when trying to create bucket during rebalance

Because it's a temporary condition thus 400 is not appropriate.

Change-Id: Ib37adee1353c4c2837a0e6dee93728c7102af9e7
Orig-Reviewed-on: http://review.couchbase.org/13016
Reviewed-on: http://review.couchbase.org/13279
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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