send buildpack_key or buildpack_git_url in staging message

keep "buildpack" key in staging message for now, we intend to remove
it once the matching dea code is deployed.

use buildpack_key to send admin buildpacks to use to the DEA
(rather than using the CC URL of the admin buildpack)

don't send buildpack_key when specifying a url and vice versa

disallow special characters in admin buildpack names