refs #4521 - Openstack Compute Resource: Boot from Volume on new Host Creation

At my company we use all Ceph storage volumes for VM storage. This required the ability to boot from these volumes which is not currently available with any Foreman version.

The code changes allow creating a boot volume from specified image and attaching to host via the block_device_mapping v2 api call in the latest fog commits: 

https://github.com/fog/fog/commit/a1eb38d1b980d983ce94ac18b1592183df92571d

Please let me know if you see any glaring mistakes...I'm just getting familiar with Ruby on rails.
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