lightdm: Use lightdm based on tag in hostconfig

Lightdm is enabled per host by providing the 'lightdm' tag in our
hostconfig.

Installation steps on distmaster:

 - install updated gdm.service (see mariux64/mxtools)
 - bee install lightdm-1.26.0-0.x86_64 (surprise:)
 - systemctl enable lightdm

Conditions and installation tested on sigstkflt, sigill & sigint.

lightdm.be0

@@ -76,6 +76,7 @@ mee_install_post() {
 	Description=LightDM
 	Wants=nvidia.service
 	After=nvidia.service
+	ConditionPathExists=/node/tags/lightdm
 
 	[Service]
 	ExecStart=/usr/bin/lightdm