autogenerate a signing key if not specified
autogenerate a signing key if not specified

this is purely for developer convenience; tokens will only be valid for
this one ATC and only until it restarts. you should set the same fixed
signing key for all ATCs in production. (the BOSH release automates
this.)

zachgersh/atc,zachgersh/atc,zachgersh/atc,zachgersh/atc