Allow users to specify a tmp location via config

Fixes bug 981104

Although the temporary directory used can be controlled via
environment variables, this patch provides a way to define it
explicitly via a config option.

The default value is None, which behaves per the doc below:
http://docs.python.org/2/library/tempfile.html#tempfile.tempdir

Flags: DocImpact
Change-Id: I47b6d8bac734f453c80d541b46086a30e847c859
Merge "Allow users to specify a tmp location via config"