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: I6cd786478fc2895882f22f7c443c077a9e95fcea
Reviewed-on: http://review.couchbase.org/35579
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>