fix launch: path=None unless using image service

A bug was introduced in 7c1f27b67d53c97ef0eb417564ba16df70db1979
wherein the base_ image directory was always given as the path
argument to to vms.management.Management.launch, thereby overriding
VMS_DISK_URL. Since the path argument to launch tells it where to look
for artifacts (descriptor files, base disks, etc.), we only want to
set it to nova's base_ directory when we're using glance to store
bless artifacts.

Change-Id: I59261b4c99f03fb2c129077d9e5b7538bd547b42