MB-5904 remove passwords from rebalance log output

- remove obvious password logging
- remove password from the ebucketmigrator_srv state
- remove password from the ns_rebalance_observer state

Change-Id: I32a173311b2c8e195b0ebbfb2642008d31eb892e
Reviewed-on: http://review.couchbase.org/28156
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>