implement warming in new cli tool

Adds infrastructure for calling the legacy PHP scripts. As the
CONST_* values cannot be set from the python script, hand the values
in via secret environment variables instead. These are all
temporary hacks for the transition phase to python code.

nominatim/config.py

@@ -0,0 +1,39 @@
+"""
+Nominatim configuration accessor.
+"""
+import sys
+import os
+
+from dotenv import dotenv_values
+
+class Configuration:
+    """ Load and manage the project configuration.
+
+        Nominatim uses dotenv to configure the software. Configuration options
+        are resolved in the following order:
+
+         * from the OS environment
+         * from the .env file in the project directory of the installation
+         * from the default installation in the configuration directory
+
+        All Nominatim configuration options are prefixed with 'NOMINATIM_' to
+        avoid conflicts with other environment variables.
+    """
+
+    def __init__(self, project_dir, config_dir):
+        self._config = dotenv_values(str((config_dir / 'env.defaults').resolve()))
+        self._config.update(dotenv_values(str((project_dir / '.env').resolve())))
+
+    def __getattr__(self, name):
+        name = 'NOMINATIM_' + name
+
+        return os.environ.get(name) or self._config[name]
+
+    def get_os_env(self):
+        """ Return a copy of the OS environment with the Nominatim configuration
+            merged in.
+        """
+        env = dict(os.environ)
+        env.update(self._config)
+
+        return env