Let devstack to decide about swift storage

Besed on several discussion, the right swift storage size value should
be configured by devstack and not by devstack gate.

Fixing bug 1225664

Change-Id: I421d8ee6fc6cbde463592134832d169f91d8a91b

devstack-vm-gate.sh

@@ -160,10 +160,6 @@ EOF
         # The 24G setting is expected to be enough even
         # in parallel run.
         echo "VOLUME_BACKING_FILE_SIZE=24G" >> localrc
-        # The volume-upload-image test requires a volume to be
-        # uploaded in glance which in turns is configured for use
-        # swift as backend
-        echo "SWIFT_LOOPBACK_DISK_SIZE=2000000" >> localrc
         # in order to ensure glance http tests don't time out, we
         # specify the TEMPEST_HTTP_IMAGE address to be horrizon's
         # front page. Kind of hacky, but it works.