unattended_install.cfg: Add previously hidden image_copy_on_error param

The param image_copy_on_error tells virt-test that
when an unattended install fails, copy an already
installed guest image from a NFS mount. However,
not all test grids will have such a NFS mount configured,
so just assuming by default that such a mount is configured
is wrong.

This param was introduced on a previous commit, and it
is not available anywhere in the default config files.
Add it and set it by default to "no".

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Move image_copy test to virttest.utils_test

The namespace autotest.virt.tests is not available
when running virt-tests from outside autotest. So
move image_copy to utils_test, and use it from there.

This commit moves the code to the right places and
changes the imports accordingly.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
unattended_install: Fix message wording

The messages assume images will be copied from an
NFS mount in case of unattended install failure,
when said mount may not even exist. Let's fix the
message and notify the image copy only if the grid
owners actually configured it to do so.

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