MB-10415 hashing of admin and read only user passwords

the storage format for both credentials was unified and now is:
{User, {password, {Salt, Mac}}}

the password hashing and new storage format are enabled in 3.0
clusters

Change-Id: I3a6f5a48b2db349acbbebe76917bc6c5d8b8b977
Reviewed-on: http://review.couchbase.org/34789
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>