Assign `authority` field when generating vbucket config

This is used by comparison functions later on to determine if the config
has changed. The need for this change was detected when trying to
reconnect to a dead memcached server (using memcached:// ), upon which
the library crashed.

Change-Id: If7ed310724ce1438ed3886fb82bd55947f856d83
Reviewed-on: http://review.couchbase.org/42475
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>