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.yaml

@@ -3,6 +3,9 @@
 # For easier management of items to enable/disable,
 # use one test/module on each line, followed by a comma.
 #
+# If no variable is to be set, add 'empty: true', as the
+# 'variables' dict cannot be empty.
+#
 # Example:
 #
 # disabled_modules: >-
@@ -12,9 +15,8 @@
 #
 ---
 variables:
+  empty: true
   # ipa_enabled_modules: >-
   # ipa_enabled_tests: >-
-  ipa_disabled_modules: >-
-    dnsconfig,
-  ipa_disabled_tests: >-
-    test_dnsconfig_forwarders_ports
+  # ipa_disabled_modules: >-
+  # ipa_disabled_tests: >-