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.

tests/azure/templates/variables_centos-7.yaml

@@ -0,0 +1,19 @@
+#
+# Variables must be defined as comma separated lists.
+# For easier management of items to enable/disable,
+# use one test/module on each line, followed by a comma.
+#
+# Example:
+#
+# disabled_modules: >-
+#   dnsconfig,
+#   group,
+#   hostgroup
+#
+---
+variables:
+  empty: true
+#   ipa_enabled_modules: >-
+#   ipa_enabled_tests: >-
+#   ipa_disabled_modules: >-
+#   ipa_disabled_tests: >-