As infra uses public cloud to run jenkins jobs, some cloud
provider do not have large disk space, for example, only 20GB
root disk, not fit for k8s bay instance space requirements.
According to infra core said, if two instances are booted,
each with 10GB, it could all meet in back end clouds.
So we need to create specific flavor for magnum use. It is
verified in local env 8GB can work, so we will use 8GB now.
Closes-Bug: #1484772
Change-Id: I159623f77ad7e3083a5296b0ccc7d65cfb57e001
644cfdf8b5