In utils.tempdir, pass CONF.tempdir as an argument.

It's ugly, and potentially racy, to mess around with other modules'
global variables.  Instead, pass CONF.tempdir into tempfile.mkdtemp
as the 'dir' keyword argument.

Because we already pass **kwargs to mkdtemp, inspect **kwargs and
only set 'dir' if it's not already there.

Change-Id: I8a2b34cd051919db29facabc1664cf357073b1d7
Merge "In utils.tempdir, pass CONF.tempdir as an argument."