fix(systemd): Skip mounting /media in containers.

When running in a container it is up to the outside world to set up
mount points appropriately. This allows the user to set up a config
drive prior to booting the CoreOS container.
fix(motdgen): Fix accidental logic inversion.

Missed the ! when switching from checking $?.
Merge pull request #95 from marineam/container

Skip mounting /media in containers and motdgen fix