When using virtual media, restarting ironic-python-agent
leads to errors like 'File exists'. This is because
we use static directory (/vmedia_mnt) as mount path
and we don't remove it. It's better to use mkdtemp to create
a temporary directory for mounting virtual media
device.
Change-Id: Ibb4c3d3a2024cf51ef198bd396fb9e86ad72a320