Use DATABASE_URL by default

See rails#13463 (comment) for full conversation.

railties/lib/rails/generators/rails/app/templates/config/databases/frontbase.yml

@@ -26,4 +26,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/ibm_db.yml

@@ -64,4 +64,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/jdbc.yml

@@ -56,4 +56,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/jdbcmysql.yml

@@ -28,4 +28,4 @@ test:
 
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
-production: <%%= ENV["RAILS_DATABASE_URL"] %>
+production: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/jdbcpostgresql.yml

@@ -45,4 +45,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/jdbcsqlite3.yml

@@ -21,4 +21,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/mysql.yml

@@ -39,4 +39,4 @@ test:
 #   mysql2://myuser:mypass@localhost/somedatabase
 #
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/oracle.yml

@@ -35,4 +35,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/postgresql.yml

@@ -66,4 +66,4 @@ test:
 #   postgres://myuser:mypass@localhost/somedatabase
 #
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/sqlite3.yml

@@ -27,4 +27,4 @@ test:
 #   sqlite3://myuser:mypass@localhost/full/path/to/somedatabase
 #
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/lib/rails/generators/rails/app/templates/config/databases/sqlserver.yml

@@ -45,4 +45,4 @@ test:
 # Do not keep production credentials in the repository,
 # instead read the configuration from the environment.
 production:
-  url: <%%= ENV["RAILS_DATABASE_URL"] %>
+  url: <%%= ENV["DATABASE_URL"] %>

railties/test/application/rake/dbs_test.rb

@@ -21,16 +21,9 @@ def database_url_db_name
       end
 
       def set_database_url
-        ENV['RAILS_DATABASE_URL'] = File.join("sqlite3://:@localhost", database_url_db_name)
+        ENV['DATABASE_URL'] = File.join("sqlite3://:@localhost", database_url_db_name)
         # ensure it's using the DATABASE_URL
         FileUtils.rm_rf("#{app_path}/config/database.yml")
-        File.open("#{app_path}/config/database.yml", 'w') do |f|
-          yaml = <<-YAML.strip_heredoc
-            #{ENV['RAILS_ENV']}:
-              url: <%= ENV['RAILS_DATABASE_URL'] %>
-          YAML
-          f << yaml
-        end
       end
 
       def expected