17175: Wait for seedDatabase before starting railsapi.

17175: Wait for seedDatabase before starting railsapi.

If the rake task and the app start concurrently, they both try to set
up the database seeds. The exclusive-locking mechanism in
services/api/app/lib/current_api_client.rb doesn't cover this case
because the rake task and app don't use the same cache. If the app
commits the database seeds between the rake task's "check existing"
and "commit" steps, commit fails and the rake task aborts.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>