Added systemd support for managing services.

NOTES:
*  in order to mimic the behaviour of upstart services, systemd services **must be enabled** in order to be considered "installed".
*  as of this PR, manipulating systemd services requires root privileges (as systemd was indended to be used all along, and as upstart also is). I have [another branch](https://github.com/aznashwan/juju/tree/systemd-user) where I have already made the couple of adaptations required to use this in user mode should we so wish it; however, as of now, the version of dbus currently available in CentOS 7 has a bug in which it fails to set an environment variable that systemd relies on during operations in user mode...

Am looking forward to hearing what you guys think about the system vs. user mode topic and am open to any suggestions in that regard.
Until then, happy holidays and see you all in 2015 :smile: