MB-7091: Check if ns_server is ready to create bucket

When creating a bucket, need to check if ns_server is up and ready.
If not, we need to wait and try again until it is ready.

Change-Id: Ieff385fc2889bb6f8b9323bdc6be185b8d7e9b3e
Reviewed-on: http://review.couchbase.org/22379
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Matt Ingenthron <matt@couchbase.com>