Partial Fix LP #992640 - Extend volume group

Tempest's volume list tests create a number of 1G
volumes, but devstack's default volume backing file
size is only 2G. If tempest is being executed, we
set VOLUME_BACKING_FILE_SIZE=4g in localrc, enabling
Tempest's volume tests to succeed without cryptic
error messages...

Change-Id: I3f9555095f2c6a86e7b5984cbc209d7e4235c567

devstack-vm-gate-host.sh

@@ -86,6 +86,10 @@ if [ "$DEVSTACK_GATE_TEMPEST" -eq "1" ]; then
     # We need to disable ratelimiting when running
     # Tempest tests since so many requests are executed
     echo "API_RATE_LIMIT=False" >> localrc
+    # Volume tests in Tempest require a number of volumes
+    # to be created, each of 1G size. Devstack's default
+    # volume backing file size is 2G, so we increase to 4G
+    echo "VOLUME_BACKING_FILE_SIZE=4G" >> localrc
 fi
 
 # The vm template update job should cache some images in ~/files.