Revive collection unit tests

Seemingly, ansible-test unit tests were not run in CI for a while and
are broken in multiple places.

This patch aims to re-incarnate unit testing of modules
and execute them as part of the sanity tests.
We use sanity right now, as they perform testing against different
ansible versions and unit tests take multiple seconds to complete.
So spawning a separate nodes would be an overkill.

Unit tests have also detected a regression in server module, as existing
nets generation can create a generator, where `isinstance(net, dict)`
will return negative result, as it is gonna
be a generator, not a dict.
So this part has been refactored to avoid poitential regressions.

Change-Id: Ibefc087239b5c8d5e843c977c2c5a2250ae0bfbd
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>

tools/run-ansible-sanity.sh

@@ -44,3 +44,7 @@ ansible-test sanity -v \
     --python ${PY_VER} \
     $SKIP_TESTS \
     plugins/ docs/ meta/
+
+ansible-test units -v \
+    --venv \
+    --python ${PY_VER}