Missing validator

This patch adds validator to CONFIG_SWIFT_STORAGES to make clear
that values have to be paths only (without IP addresses).

Change-Id: Ia5327ac2df0e8123f75934fdb3ae21dcf839f69c
Fixes: rhbz#1128303
(cherry picked from commit b04f4ace822e2f050346a7f2ba588de4cba4f70d)