Allow providing the environment variable to load configuration from.

Now, `config_master`, `config_minion` and `client_config` accept the proper environment variable to load configuration from. This allows projects sub-classing salt's master and minion, for example, to provide their own environment variable.
Protect against the `log_path` being set to `None`.
Merge pull request #3332 from s0undt3ch/develop

Some more external sub-classing and code re-use support