Assist with fleet/docker dependency issues

[lfittl]

moby/moby#7228 (comment)
moby/moby#6791 (comment)
https://groups.google.com/forum/#!msg/coreos-dev/qbVrYpDUtG8/t88RbGD_assJ
https://coreos.com/docs/launching-containers/launching/getting-started-with-systemd/

The issue is that systemd state does not reflect docker state (i.e. container isn't actually ensured to be running), and additionally does not ensure that the service is actually available.

e.g. the database needs sometime to start up until it can actually accept connections.

systemd should allow mark that dependency as fulfilled once the service is actually available.