Introduce "static/env.config.php" for environment variable mapping to config cache entries

- Added new database.port config value (used for MYSQL_PORT)
- Removed now obsolete db environment variable functionality
- Added functionality to load env variables (overwrites DB based cached)

static/defaults.config.php

@@ -32,6 +32,11 @@ return [
 		// Can contain the port number with the syntax "hostname:port".
 		'hostname' => '',
 
+		// port (Integer)
+		// Port of the database server.
+		// Can be used instead of adding a port number to the hostname
+		'port' => null,
+
 		// user (String)
 		// Database user name. Please don't use "root".
 		'username' => '',