cluster template image update does not update cluster_distro

[Ricardo Rocha]

Here's an example sequence:

$ magnum cluster-template-create --name rocha-test00 --image-id c60d724b-8bf8-416d-9b3f-68e17b733347 --external-network-id CERN_NETWORK --flavor-id m2.medium --master-flavor-id m2.medium --coe swarm
...
| cluster_distro | fedora-atomic |
| image_id | c60d724b-8bf8-416d-9b3f-68e17b733347 |
...

Which is correct:
$ glance image-show c60d724b-8bf8-416d-9b3f-68e17b733347 | grep os_distro
| os_distro | fedora-atomic |

If i then update the cluster template image with the following (different os_distro):
$ glance image-show c4537d3b-9c90-4620-b802-c1064df9f40e | grep os_distro
| os_distro | fedora-atomic-staging

$ magnum cluster-template-update rocha-test00 replace image_id=c4537d3b-9c90-4620-b802-c1064df9f40e
...
| cluster_distro | fedora-atomic |
| image_id | c4537d3b-9c90-4620-b802-c1064df9f40e |
...

The cluster_distro is not properly updated with the value of the new image.

This causes issues as currently the driver is selected based on a combination of cluster_distro and coe.