Add support for creating git repos which support pull/push over SSH.

* Git repos have their own unit file and create a user if missing
* Extended gen-auth-keys command to support repositories
* Updated githost image to run and provide git tools
* Update switchns to support git and git-readonly based operations
* Update locations that were missed in gears->containers move
* Fix container PID detection for docker-native driver
* Moved simple case of systemd unit/slice/target creation into utils