Add os-block-device-mapping to v3 API

Create a new v3 extension called os-block-device-mapping to handle
the block devices specified on server creation. This extension was
os-block-device-mapping-v2-boot in the old API.

Only the new block device mapping syntax has been ported although the
old one can still be used with v2 API.

blueprint: improve-block-device-handling

Change-Id: I898ae56539c6101680983d1eca4bce6d92bd5ce6
Merge "Add os-block-device-mapping to v3 API"