settings: Resolve all but one Django 1.7 warning

Resolve remaining Django 1.7 warnings with the exception of a single
deprecation warning. However, this remaining issue is really a
non-issue due to the limited (and non-offending) use of the code.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>