Increase default swift storage

Swift storage is used as glance image back-end. Tempest have cinder to
uploads 1 GiB image from cinder in twice.

In parallel execution in cause an issue, bacuse the current default size is
1_000_000 KiB.

Increasing the default swit storage size from 1_000_000 KiB 4_000_000
KiB when tempest is enabled.

Fixing bug 1225664

Change-Id: Iccd6368e4df71abb5ccfe7d361c64d86e1071d35