devstack: add support for rescue mode
devstack: add support for rescue mode

This adds config for setting rescue interface for node,
rescue network, and rescue images.

Change-Id: I332223f187783fc06e19e62a886a8a7378c62cf2
Partial-Bug: 1526449
Co-Authored-By: Mario Villaplana <41f440442dc92dc4343f515881fae72fb87d2637@gmail.com>
Co-Authored-By: Annie Lezil <f7e2fe471a6db5cc9a3ce1b49b8bfd4505fba058@gmail.com>

openstack/ironic,openstack/ironic