cleanup(inventory): Drop support for multiple connections

The support for connections to multiple clusters in the inventory
plugin is dropped to better align with user expectations and how other
inventories work. If inventories of multiple clusters are needed the
inventory can be run multiple times with different configurations.
This also helps to clean up the code and make it simpler.

For now this adds a compatibility helper so that configurations with a
single connection entry remain supported and a warning is emitted.

Signed-off-by: Felix Matouschek <fmatouschek@redhat.com>

examples/inventory.kubevirt.yml

@@ -1,7 +1,6 @@
 ---
 plugin: kubevirt.core.kubevirt
-connections:
-  - namespaces:
-      - default
-    network_name: bridge-network
-    label_selector: app=test
+namespaces:
+  - default
+network_name: bridge-network
+label_selector: app=test

examples/kubesecondarydns.kubevirt.yml

@@ -1,7 +1,6 @@
 ---
 plugin: kubevirt.core.kubevirt
-connections:
-  - namespaces:
-      - default
-    network_name: bridge-network
-    kube_secondary_dns: true
+namespaces:
+  - default
+network_name: bridge-network
+kube_secondary_dns: true

examples/services.kubevirt.yml

@@ -1,6 +1,5 @@
 ---
 plugin: kubevirt.core.kubevirt
-connections:
-  - namespaces:
-      - default
-    use_service: true
+namespaces:
+  - default
+use_service: true