MB-5904 - change ns_config format of rouser credentials

from {"user","password"} to {"user", {password, "password"}}
so existing sanitization works for this password too

otherwise I will have to write special handling
for messages like this one:

config change:
read_only_user_creds ->
{"ruser","asdfghytr"}

Change-Id: Ie8e7892fca077f7f54abbee027234b3b939136d3
[alk@tut.by: fixed no-auth case]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/28270
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>