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

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

Change-Id: I900458af3107c5fccd66aa8732b6c0a05ee641ab
Reviewed-on: http://review.couchbase.org/13016
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>
Reviewed-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>

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