Database: support load balancing reads in Rails

Add support for the configuration of read-only query load balancing as a part of Rails.

Implements (at a global level), the support for configuration of https://docs.gitlab.com/ee/administration/database_load_balancing.html

Added:

• checkConfig to ensure that internal PostgreSQL is not being installed
• global.psql.load_balancing properties.
• RSpec to test checkConfig
• RSpec to test configuration renders as expected.
• Examples under examples/database/values-loadbalancing-*

Tested configuration

NOTE: Properties were added here in snake_case to directly align to upstream documentation of these properties. They can be swapped in the future, but it may be time to re-evaluate the choice of camelCase.

Closes #1004 (closed)