qa_devstack: Add parameters for extra disk setup

The CI job uses a cleanvm instance where the root disk has limited
space. But another disk is available for usage. With the new parameters
EDISK_DEV and EDISK_TARGET this extra disk can be setup and used.
would still be nice to use the $1 instead of /dev/vdb here (and pass it as a parameter) so that the calling code can easily be updated to a different device name. but not a blocker for me, can still be done later.