Add env var for choosing of deploy image element

Make sure we can easily switch between deploy and
deploy-kexec image elements when building a deploy
ramdisk by setting the environment variable.

Default to deploy, because deploy-kexec is not reliable
right now and our production-like cloud (TripleOCloud)
has switched to deploy.

Change-Id: Ia9dc58a5de6a7677824eebc82d13888355378b77