Use configurable driver name in lookup URL

Allow configuration via command line arguments or kernel parameters.
Default to agent_ipmitool, the reference driver.

Depends on https://review.openstack.org/#/c/84795/12

Change-Id: I55c4a8713308d038002a6567471cd862bf89ec76
(cherry picked from commit 80c620626136f329457a8ea8cf84a21886048937)
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.

Change-Id: Id8af9be920ba51e7e1ce60f4ffd1477e413582c9
(cherry picked from commit 0cfa2e71b7e35dd750dd1b9f0804d40d84ba418d)