fix(cros_sdk): Hack around systemd's new restriction on cgroups.

This whole pile of code needs to go away at some point but this is a
quick hack for now. If /sys/fs/cgroup is mounted ro we need to remount
it as rw.
Merge pull request #16 from marineam/rw

fix(cros_sdk): Hack around systemd's new restriction on cgroups.