tests: janitorial: cleanup temporary directories

A few tests don't cleanup the temporary directories they created to run.
A the end of the day, you can easily end up with tons of
entries in /tmp.

This patch ensures each testcase does its own cleanup, by using
a new contextmanager to handle and clean automatically a
temporary directory.

Also, move away the test data handling from ifcfgConfigWriterTests's
__init__/__del__ and move it into setUp and tearDown, to make the
process more predicatable and robust.

Change-Id: If6581771479de9f72360ea537de32abcdad496b3
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22377
Reviewed-by: Dan Kenigsberg <danken@redhat.com>