Accept new parameters for `prepare_image`

The parameters sent to `prepare_image` changed in
https://review.openstack.org/#/c/86490/

This patch brings `prepare_image` up to date with that change.
It also changes the way configdrive is written to disk, to match
that Ironic is now allowing Nova to build an ISO partition and
send the raw image to the agent.

This patch also swaps out subprocess.call for processutils.execute
in the standby module, since the commands were being changed anyway.

Lastly, this patch changes the expected `hashes` dict to be a
string parameter called `checksum`, to match what glance returns.

Change-Id: Id8af9be920ba51e7e1ce60f4ffd1477e413582c9
(cherry picked from commit 25653fb8f38b1431c2b956227e3848f8306dc7b2)