Make sure tempest directories exist when modifying

* devstack-vm-gate.sh: In stable/grizzly and earlier, the
$BASE/new/tempest, /opt/stack/data/tempest and
/opt/stack/new/devstack/files directories may not exist. Look before
you leap.

Closes-Bug: #1235303
Change-Id: Ife6050e82e04b9fd50514d6d65a6dc438661dbf8