qa_devstack: Use extra disk if available
qa_devstack: Use extra disk if available

The CI job uses a cleanvm instance where the root disk has limited
space. But another disk is available for usage. Use this disk (/dev/vdb)
if available.

aspiers/automation,gosipyan/automation,gosipyan/automation,aspiers/automation,aspiers/automation,vmoravec/automation,SUSE-Cloud/automation,vmoravec/automation,SUSE-Cloud/automation,SUSE-Cloud/automation,vmoravec/automation,gosipyan/automation,aspiers/automation,vmoravec/automation,SUSE-Cloud/automation,gosipyan/automation