[Discovery]Update database config to depend on DATABASE_URL

[pgwillia]

We've introduced password environment variables in three places config/database.yml and docker-compose.lightweight.yml/docker-compose.yml and .env_deployment_sample. We should simplify this scheme to use DATABASE_URL/DISCOVERY_DATABASE_URL instead which will allow more flexibility for how the database is setup. See https://github.com/ualbertalib/jupiter/blob/add9c58c6b8c9642931a2e58e296c8ce3bd923c6/docker-compose.yml#L61 and

discovery/config/database.yml

Lines 12 to 18 in af49bd0

	default: &default
	adapter: mysql2
	encoding: utf8
	pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
	username: root
	password: <%= ENV['DISCOVERY_DATABASE_PASSWORD'] || 'mysecretpassword' %>
	host: 127.0.0.1

Originally posted by @pgwillia in #1591