virttest.data_dir: Delete everything inside tmp dir, not the dir itself

The way SELinux setup is dealt with makes re-creating
the tmp dir every time a problem. So clean inside of
said dir, rather than delete it entirely.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
unattended_install: Make image_backup = yes by default

Meaning successful unattended installs to create backup
copies, which prevents non-jeos runs to backup bad
images at the beginning of tests and apply them on
subsequent tests. People running unattended installs
on their grids can override that param if they want
anyway.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.env_process: Fix backup_image behavior

Using the test runner defaults, instead of backing up
an image at the end of an unattended install, we'll
end up restoring a (potentially bad) backup instead.

So if backup_image is set to 'yes', ignore restore_image.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>