iRMC OOB rescue mode support

This proposes to support OOB rescue mode by booting rescue image from
virtual media.
Supporting OOB rescue mode provides customer with the same network
configuration as costumer's production environment which is booted from
Cinder [1] in customer's tenant network.

If rescue mode booted from rescue network using Pluggable network
providers [2], IB rescue mode provides customer with NOT same network
configuration as costumer's production environment.
This would cause another problem when customer investigated problem in
their production environment.

Therefor it is better to support not only IB rescue mode [3], but also OOB
rescue mode.

[1] https://blueprints.launchpad.net/ironic/+spec/volume-connection-information
[2] https://blueprints.launchpad.net/ironic/+spec/network-provider
[3] https://blueprints.launchpad.net/ironic/+spec/implement-rescue-mode

- Nov 9th, 2015, Naohirot:
Overall idea can be found in the following presentation slides P.56 - P.58.

"Ironic Towards Truly Open and Reliable, Eventually for Mission Critical"
 http://www.slideshare.net/naohirot/ironic-towards-truly-open-and-reliable-eventually-for-mission-critical
 https://www.youtube.com/watch?v=MpSqDA3jo0I