Decouple DB config loading from rails codebase

gems/gitlab-backup-cli/lib/gitlab/backup/cli.rb

...ackup-cli/lib/gitlab/backup/cli/context/source_context.rb

gems/gitlab-backup-cli/lib/gitlab/backup/cli/database.rb

...ackup-cli/lib/gitlab/backup/cli/database/configuration.rb

...b-backup-cli/lib/gitlab/backup/cli/database/connection.rb

...ackup-cli/lib/gitlab/backup/cli/database/each_database.rb

...lab-backup-cli/lib/gitlab/backup/cli/database/postgres.rb

...tlab-backup-cli/lib/gitlab/backup/cli/database/wrapper.rb

...tlab-backup-cli/lib/gitlab/backup/cli/targets/database.rb

...gitlab-backup-cli/lib/gitlab/backup/cli/tasks/database.rb

...ackup-cli/spec/gitlab/backup/cli/targets/database_spec.rb