Code review comment for ~chad.smith/cloud-init:clean-status-commands

Addressed review comment, shuffling uses_systemd out of Distros.uses_systemd and into a standalone function. Also per our discussions in IRC about systemd commandline utilities shortcomings on <= Zesty, we can no longer look at "systemctl is-enabled cloud-init.target". Using your suggestions @ http://paste.ubuntu.com/26067095/, we now just check presence of /etc/cloud/cloud-init.disabled, kernel cmdline contianing "cloud-init=disabled" or absence of /run/cloud-init/enabled to determine whether cloud-init is disabled.