Fix misc issues with os-multinic v3 API extension

Fixes action names for the extension to be consistent with other
extensions

Replaces WebDav UnprocessableEntity return code with BadRequest
when the client body is bad

Cleanup exception handling

Partially implements blueprint nova-v3-api

Change-Id: Ied433fe6cf93d6724201035120612036dec53283
Merge "Fix misc issues with os-multinic v3 API extension"