Set default snap channel on charms to 1.7 stable

cluster/juju/layers/kubernetes-e2e/config.yaml

 options:
   channel:
     type: string
-    default: "stable"
+    default: "1.7/stable"
     description: |
       Snap channel to install Kubernetes snaps from

cluster/juju/layers/kubernetes-master/config.yaml

@@ -23,7 +23,7 @@ options:
       detected on a worker node.
   channel:
     type: string
-    default: "stable"
+    default: "1.7/stable"
     description: |
       Snap channel to install Kubernetes master services from
   client_password:

cluster/juju/layers/kubernetes-worker/config.yaml

@@ -22,7 +22,7 @@ options:
       switch to privileged mode if gpu hardware is detected.
   channel:
     type: string
-    default: "stable"
+    default: "1.7/stable"
     description: |
       Snap channel to install Kubernetes worker services from
   require-manual-upgrade: