PostgreSQL: restore per sub-chart configuration

Summary

Restore the ability to configure PostgreSQL per sub-chart.

Details

All items of global.psql can be overridden, except .ssl: {}

The following will inherit from the global, if not present:

• psql.password.secret
• psql.password.key
• psql.port (pre-existing behavior, fine to remove)

The following will never inherit from global:

• psql.load_balancing

Two examples have been added:

• examples/database/values-per-service.yaml
• examples/database/values-per-service-loadbalancing.yaml

Testing has been added, documentation remains to be done.

Testing

Database configuration
  When using per chart configuration
    when separate configuration is provided for Sidekiq
      configure webservice with "global", sidekiq with "other"
    when separate configuration is provided for Webservice
      configure sidekiq with "global", webservice with "other"
    when overriding only host for Webservice
      only host is overridden
    when setting global password, it is inherited when not overridden
      password is inherited, if not specified
    when load_balancing is used
      load_balancing is configured globally, not by Webservice
        load_balancing is not inherited by Webservice, but used by Sidekiq
      load_balancing is configured globally, and for Webservice
        separate load_balancing is used by Webservice and Sidekiq
      load_balancing is configured only for Webservice
        load_balancing is only used by Webservice

Closes #1812 (closed)