When juju 3.x is used (installed from a snap in strict confinement) the
temporary directory of the system (e.g. /tmp) can't be read by 'juju',
this is used by zaza-openstack-tests to write testing policy files
among other transitory files that are passed to juju's CLI.
This change moves Python's default temporary file to ~/tmp unless
overriden by the environment variable TEST_TMPDIR.
