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