fix(seed-deis-registry): move into deis-registry.service

Since we moved seed-deis-registry from the user-data file
in #770, the Makefile broke since we cannot perform many
systemctl actions on a oneshot service. This moves it into
an ExecStartPost after the deis-registry service starts.

fixes #774
Merge pull request #775 from opdemand/seed_registry_fix

fix(seed-deis-registry): move into deis-registry.service
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