Configurable USER_CONF_ROOT path

[ScottWales]

Problem

The USER_CONF_ROOT path is used in a variety of places for storing logs, secrets and the Jupyter database. While most uses are configurable in jupyter_config.py, the output path of RotatingUISFileHandler is not. We'd like to place this log in a different location with append-only permissions.

Proposed Solution

• Allow USER_CONF_ROOT to be configured with an environment variable, e.g. $CYLC_CONF_PATH in the same way as cylc-flow.

and/or

• Add a new configuration item e.g. c.CylcUIServer.server_log_path that sets an output path for RotatingUISFileHandler (defaulting to USER_CONF_ROOT/log as it currently does).

I can see a complication that RotatingUISFileHandler is initialised very early in the startup, potentially before the configuration is loaded.