teuthology/orchestra: store temporaries in tmp subdir

teuthology/orchestra: store temporaries in tmp subdir

Due to protections turned on in Ubuntu 20.04 from [1], files created by
other users (e.g. ubuntu) can not be manipulated by other users, even
root.

Instead of using /tmp as the parent directory for `mktemp` files, use a
sub-directory under it instead.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=30aba6656f61ed44cba445a3c0d38b296fa9e8f5

Fixes: https://tracker.ceph.com/issues/51169
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>