Make secrets source a variable

Secrets in the Mozilla world come from a secret module that only Mozilla
can see. We introduce a variable for the source of secrets to allow
secrets to come from the fakemozilla environment.
Add fake SSH keys for SSH server

These SSH keys are valid. They came from a Vagrant machine somewhere.
Add fake rsa key for mirror

With this patch, hg:ssh now applies cleanly in the fakemozilla Vagrant
environment \o/