Configure Hue ini-file directory via environment variable

Currently, Hue loads configuration data only from <hue_root>/desktop/conf. In the packaging, that directory is symlinked to /etc/hue. To enable re-configuration more dynamically, let's support an environment variable ($HUE_CONF_DIR). This is in the same vein as how Hadoop does it.

An option would have been to use command-line flags, but they'd need to be added to all management commands and other sundry things, and this is an infrequent enough use case that I'd like to avoid doing that.