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
* Added new command auth-keys-command and executable gear-auth-keys-command to support SSHAuthKeysCommand based login
* Fixed ssh to container (container name is container id, instead of user login)
* Fixed clean so it stops containers/slices before disabling them
Merge pull request #76 from kraman/git

[Pls review] Add support for creating git repos which support pull/push over SSH.