packaging: setup: iso domain check on / dir

previously specifying a non existent directory with
parent directory '/' like '/ISOstorage' caused a
loop trying to get the base path issuing an error.

Now '/' is handled and it is possible to define
directories like '/ISOstorage' for iso domains

Change-Id: I0aec8d729438cc1c19ba20644aa051fb70ff615f
Related-To: https://bugzilla.redhat.com/1004005
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>