MB-100: ensure writability of paths more carefully

Particularly we check directory existence more carefuly, without
trying to create it.

And we also don't try to enforce that all ns_server paths have to be
writable. Only tmp is something we try to create.

Change-Id: Ib9fdabe17010bd838e1004c723adeb275407a991
Reviewed-on: http://review.couchbase.org/29119
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>