Code review comment for ~smoser/cloud-init:bug/before-fsck

This potentially slows down startup a bit, but running fsck and cloudinit.service in parallel is not safe thus the change looks good to me and I have no better idea.

The change is also consistent with cloud-init's documentation:
http://cloudinit.readthedocs.io/en/0.7.9/topics/boot.html#network