Fix minion-side key management

[Benjamen Keroack]

Even though we pretend to support independent SSH keys per repo, in fact individual managed servers can only have one active keypair (which gets overwritten with every gitdeploy initialization).

Need to investigate using SSH alias on the minion side (which is what we do daft-server side).

[Benjamen Keroack]

First draft of code is written. Needs to be debugged and verified, of course.