Support deploy keys

[pinkwah]

Allow for users to specify deploy keys for each package. The keys themselves are stored somewhere else, eg /private/f_komodo/keys

[markusdregi]

Good idea! The keys could even be kept as Jenkins secrets and persisted to a file only while doing the GitHub interrogation (using GIT_SSH_COMMAND or similar). I don't know which is better, but it leaves us at least with multiple options to consider for how to keep track of the keys...