Provide "enabled" and "disabled" methods to Upstart service module.
Provide "enabled" and "disabled" methods to Upstart service module.

This also improves a previous commit where "enable" and "disable"
would not do as expected to Upstart services. This is because the
prevous commit used "update-rc.d" exclusively which does nothing to
Upstart services.

Close #1125.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt