Bump SWIFT_LOOPBACK_DISK_SIZE_DEFAULT over swift max_file_size

Swift is returning 50x error codes because its disk is too small, set
size bigger then max_file_size in an attempt to fix the problem, or at
least reduce it.

"we create a 4GB device, but swift thinks it can write 5GB, hence fail"
--sdague

This patch based off of Iccd6368e4df71abb5ccfe7d361c64d86e1071d35

Change-Id: Ib56a98cd74e7edf1fa90facc25c72632d43180f1
Related-Bug: #1225664
diff --git a/lib/swift b/lib/swift
index b46537f..83c4ebb 100644
--- a/lib/swift
+++ b/lib/swift
@@ -59,9 +59,9 @@
 # kilobytes.
 # Default is 1 gigabyte.
 SWIFT_LOOPBACK_DISK_SIZE_DEFAULT=1G
-# if tempest enabled the default size is 4 Gigabyte.
+# if tempest enabled the default size is 6 Gigabyte.
 if is_service_enabled tempest; then
-    SWIFT_LOOPBACK_DISK_SIZE_DEFAULT=${SWIFT_LOOPBACK_DISK_SIZE:-4G}
+    SWIFT_LOOPBACK_DISK_SIZE_DEFAULT=${SWIFT_LOOPBACK_DISK_SIZE:-6G}
 fi
 
 SWIFT_LOOPBACK_DISK_SIZE=${SWIFT_LOOPBACK_DISK_SIZE:-$SWIFT_LOOPBACK_DISK_SIZE_DEFAULT}