Fix reading strings from conf for remote logging

[jlowin]

The default remote_logging values of None and False were being read as
strings rather than Python objects and therefore misinterpreted

[landscape-bot]

Code quality remained the same when pulling 4d9f36b on jlowin:fix-remote-base-None into ab1c90b on airbnb:master.

[coveralls]

Coverage increased (+0.1%) to 67.201% when pulling 4d9f36b on jlowin:fix-remote-base-None into ab1c90b on airbnb:master.

[bolkedebruin]

Should this not be fixed in configuration.py itself? Eg. if "None" is found, we don't set a value? if != 'None' is bound to have some fix it by doing a "if is not None" in the future

[jlowin]

The issue is that I had unwittingly added remote_base_log_folder = None to the airflow.cfg template, so anyone who regenerated airflow.cfg in the last month or so has a string value "None" there. It shouldn't actually be "None" or even None (the Python object); the real default is ''. So I added that line just to cover people who installed Airflow prior to this PR going into a release.