agent: use release 'url' in ApplianceConfig

The appliance now supports using a release image URL:
openshift/appliance#352
Hence, we can now use a nighthly release url in
the ApplianceConfig template.

Author: danielerez

agent/roles/manifests/templates/appliance-config_yaml.j2

@@ -1,12 +1,8 @@
 apiVersion: v1beta1
 kind: ApplianceConfig
 ocpRelease:
-  version: {{ version }}
-  channel: candidate
-  cpuArchitecture: {{ ansible_architecture }}
+  url: {{ image }} 
 diskSizeGB: 200
 pullSecret: {{ pull_secret_contents }}
 sshKey: {{ ssh_pub_key }}
-imageRegistry:
-  uri: quay.io/libpod/registry:2.8
 userCorePass: core