remove unnecessary cleanup in nova startup

nova's instances directory may be a shared directory (ie nfs), in
these cases, we do not want to call nova_cleanup at startup since it
deletes everything under $NOVA_INSTANCES_PATH.

The nova_cleanup routine will still be called by the clean.sh script
which is fine since we're presumably cleaning up the whole openstack
cluster at that point.

Change-Id: Ieb4e5d0508d4ed4c5349c497554c5da2993c9cb0
Closes-Bug: #1649389

stack.sh

@@ -893,7 +893,6 @@ fi
 if is_service_enabled nova; then
     # Compute service
     stack_install_service nova
-    cleanup_nova
     configure_nova
 fi