Currently Devstack (and devstack gate) uses the default
max_file_size of for Swift (5GB). However the loopback file
is only 1 or 2 GB is size.
We are looking at setting a default FALLOCATE_RESERVE in swift
https://review.openstack.org/#/c/288011.
Because of this our max file size test fails due to the max_file_size
and devstacks loopback size being too small.
This patch sets this to a more sane size by default inside DevStack.
See the gerrit link above for more details.
Tempest uses a loopback of 6GB, so in this case the swift default of 5GB
should be fine.
Change-Id: If09eab7d16ae67bd252020e00e8812ff252f065b
7b85723c04