upstream ci: Add support for distro specific test configuration.

Sometimes, mostly due do differences in package versions, there are
some tests that fail on a single distribution which cannot be fixed
by ansible-freeipa, requiring that the offending package is fixed.

To keep tests running succesfully we have options to disable the
failing tests, but this changes are globally applied, meaning that, by
disabling a test, it is disable in all tested distributions.

This patch allows tests to be enabled or disabled for a specific
distribution, by setting the configuration on the 'variable' template
for the specific testing scenario.