this already exists for server rebuild, but was missing for server
create.
This option is supported from Compute API version >= 2.63, and is only
available for servers booted directly from images (not from volumes,
not from snapshots, and not from images first converted to volumes).
Additionally, this patch removes mentions of
OS_TRUSTED_IMAGE_CERTIFICATE_IDS env var from similar option help string
in server rebuild command as it is not actually implemented yet.
Change-Id: I4e9faea05c499bd91034d1d284c44fdcc8e18db5
28a376bfb0