More BFV support

The original BFV PR (#407) missed a few details. This PR fills those in. Specifically, it:
* allows the SDK user to request that the bootable volume used to create a server be deleted when the server is terminated (deleted).
* checks that the `os-block-device-mapping-v2-boot` extension is installed before attempting to use it.
* adds a code sample for creating a bootable volume.

Attn: @jamiehannaford, @gecampbell.