MB-11954: made wizard deal with with existing default bucket

We now detect if default bucket already exists. And if so we update
it, rather than try to create it.

Change-Id: I8d7a39c2efaaae83ad78f24288b0a52915270b7c
Reviewed-on: http://review.couchbase.org/40598
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>