Vagrant/Packer: Increase the template disk size to 20Gb
This also migrates the vms qemutest and staging-deposit to use it.
staging-deposit was the one detecting the original template disk size of 5Gb was not enough.
Test Plan
Following are fine:
- vagrant up qemutest
- vagrant up staging-deposit (<- no more "out of disk" space issue)
Migrated from D4236 (view on Phabricator)