enable validations in nova-lvm

As of I8ca059a4702471d4d30ea5a06079859eba3f5a81 validations
are now requried for test_rebuild_volume_backed_server.
Validations are also required for any volume attach/detach based test
in general due to know qemu issues.

This patch just turns them back on to unblock the gate.

Closes-Bug: #2025813
Change-Id: Ia198f712e2ad277743aed08e27e480208f463ac7
(cherry picked from commit 6f56c5c)
(cherry picked from commit 976364f)
(cherry picked from commit e5eb65e)

Author: SeanMooney

.zuul.yaml

@@ -258,8 +258,6 @@
         NOVA_BACKEND: LVM
         # Do not waste time clearing volumes.
         LVM_VOLUME_CLEAR: none
-        # Disable SSH validation in tests to save time.
-        TEMPEST_RUN_VALIDATION: false
         # Increase the size of the swift loopback device to accommodate RAW
         # snapshots from the LV based instance disks.
         # See bug #1913451 for more details.