MB-7961: don't try to create dbdir inside ns_bucket:config_string

Because config_string is assumed by it's callers to be pure.

Change-Id: Iec68afe519168d3cacbf0dab7378636a782e2aa0
Reviewed-on: http://review.couchbase.org/25326
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>