Allow replacing a user variable in a swift template

Prepare for a change in swift templates that will have a %USER%
variable.

Change-Id: I611ae7f82de7f2e6a38ce3de38d0600fa8687bff
Partial-Bug: 1226346
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
Merge "Allow replacing a user variable in a swift template"