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
Merge branch 'master' of github.com:openstack-dev/devstack