refactor(seed-deis-registry): remove seed-deis-registry dependency

Currently, startup takes forever because registry blocks until the
slugrunner container has been pulled. Users think things have stalled
and/or are broken - this is a bad user experience.

The ExecStartPost of the registry service already pulls the slugrunner
image, so we don't need to block until this job occurs beforehand.
Additionally, it's friendlier if the controller blocks until registry
and logger are ready - if a user attempts to make an account, all of
Deis should be up.
Merge pull request #788 from opdemand/remove_registry_block

refactor(seed-deis-registry): remove seed-deis-registry dependency
refactor(contrib/coreos): update override-plugin and user-data

This commit updates the override-plugin and user-data to better
reflect upstream, and to enable both the 1-node and n-node Vagrantfiles
to use the same user-data. This is easier to maintain, and allows
us to pull in upstream changes with greater ease.

closes #760