MB-10415 Allow to use bucket credentials in cbdocloader

... if bucket already exists
access /nodes/self that requires admin priveleges only if bucket
needs to be created

Change-Id: Ib8eb7062dc6a8cf750d97455c640eac0528c26a4
Reviewed-on: http://review.couchbase.org/35540
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>