iotests: Unify cache mode quoting

Quoting cache mode is not needed, and most tests use unquoted values.
Unify all test to use the same style.

Message-id: 20190827173432.7656-1-nsoffer@redhat.com
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>