Locate configuration with environment variable

[dmarteau]

Atm the configuration file sources.list created from the init command is located in the pluging directory. For admininstration purpose or pre-initialisation in Docker image, it could be interesting to locate this file at another place (could be an environment variable with the path of the configuration file).

[dmarteau]

Same for the location of the cache .cache_qgis_plugin_manager

[Gustry]

QGIS-Plugin-Manager will take care of following variables :

• QGIS_PLUGIN_MANAGER_SOURCES_FILE for storing a path to the sources.list otherwise, the current folder will be used.
• QGIS_PLUGIN_MANAGER_CACHE_DIR for storing all XML files downloaded otherwise, the current folder will be used .cache_qgis_plugin_manager
• QGIS_PLUGINPATH for storing plugins
• PYTHONPATH for importing QGIS libraries