CI: use current tests/ directory for upgrade testing

We should not rollback our test setup during upgrade test.
The only reason to do that would be for incompatible changes in the test
inventory, and we already checkout master for those (${CI_JOB_NAME}.yml)

Also do some cleanup by removing unnecessary intermediary variables

tests/common_vars.yml

@@ -0,0 +1,42 @@
+---
+# Kubespray settings for tests
+deploy_netchecker: true
+dns_min_replicas: 1
+unsafe_show_logs: true
+
+# Registry mirrors settings
+docker_registry_mirrors:
+  - "https://mirror.gcr.io"
+
+containerd_grpc_max_recv_message_size: 16777216
+containerd_grpc_max_send_message_size: 16777216
+
+containerd_registries_mirrors:
+  - prefix: docker.io
+    mirrors:
+      - host: https://mirror.gcr.io
+        capabilities: ["pull", "resolve"]
+        skip_verify: false
+      - host: https://registry-1.docker.io
+        capabilities: ["pull", "resolve"]
+        skip_verify: false
+
+containerd_max_container_log_line_size: 16384
+
+crio_registries:
+  - prefix: docker.io
+    insecure: false
+    blocked: false
+    unqualified: false
+    location: registry-1.docker.io
+    mirrors:
+      - location: mirror.gcr.io
+        insecure: false
+
+netcheck_agent_image_repo: "{{ quay_image_repo }}/kubespray/k8s-netchecker-agent"
+netcheck_server_image_repo: "{{ quay_image_repo }}/kubespray/k8s-netchecker-server"
+
+nginx_image_repo: "{{ quay_image_repo }}/kubespray/nginx"
+
+flannel_image_repo: "{{ quay_image_repo }}/kubespray/flannel"
+flannel_init_image_repo: "{{ quay_image_repo }}/kubespray/flannel-cni-plugin"