systemd: Don't enable systemd services when native.

It shouldn't be desired that systemd enable services when using
class native. Blanking out the SYSTEMD_PACKAGES when native seems
like the most straightforwards way to fix this problem.
systemd: Move metadata only functions to parse time.

The functions that don't have dependencies on the data in ${D} should
run at parse time. This way overrides behave correctly and the
python functions can actually be replaced/redefined in individual
recipes.

i.e:
systemd_postinst() {
}

This is useful for times when you don't want to enable or disable a
service automatically on install, but still want the rest of the
functionality of the systemd.bbclass.