bbr-atcdb job doesn't produce a valid bbr-sdk config.json when using bosh links

[dprotaso]

Bug Report

The bbr-atcdb can optionally consume some bosh links for the database

concourse-bosh-release/jobs/bbr-atcdb/spec

Lines 12 to 18 in 4679213

	consumes:
	- name: postgres
	type: database
	optional: true
	- name: concourse_db
	type: concourse_db
	optional: true

Though it doesn't read the password from the postgres link

concourse-bosh-release/jobs/bbr-atcdb/templates/config.json.erb

Lines 74 to 82 in 4679213

	if postgres_role_password.empty?
	if_link("concourse_db") do |cdb|
	postgres_role_password = cdb.p("postgresql.role.password", "")
	end
	if postgres_role_password.empty?
	raise "postgres.role.password not found through either properties or links"
	end
	end

Which is provided by the postgres release here:
https://github.com/cloudfoundry/postgres-release/blob/e19399db5e3913d4f9facbfc25b56b59e8e52f50/jobs/postgres/spec#L33-L40

This results is a config.json file without a password and backup restore fails for me

• Concourse version: see below
• Deployment type (BOSH/Docker/binary): BOSH
• Infrastructure/IaaS: Any

BOSH releases on my director (though it looks like this still applies to the newer releases)

Name                    Version  Commit Hash
backup-and-restore-sdk  1.17.2*  f7138d2
concourse               4.2.1*   0631bd9
garden-runc             1.16.3*  a19258c+
postgres                30*      0cd50c00+