fix(coreos-setup-environment): Add RemainAfterExit=yes

There is no reason for this to get re-executed if a service depending on
it comes up some time after boot. This is usually the case with Vagrant.

Noted in: https://github.com/coreos/coreos-overlay/pull/568
Merge pull request #85 from marineam/remain

fix(coreos-setup-environment): Add RemainAfterExit=yes