charm testing order can be improved

[pmatulis]

The order of testing in zaza/openstack/utilities/upgrade_utils.py determines the end-user documentation for both charm upgrades and payload upgrades. It has come to my attention that the ordering can be improved for payload upgrades, at least for OVN and Compute, as the OVN control plane will remain down until Compute is upgraded.

[pmatulis]

In a PR to the user-facing upgrade documentation (for both charms and payload) the upgrade ordering was changed. We have been trying to maintain a sync between the ordering in the latter documentation and the ordering of Zaza testing. Consider altering the testing order to reflect the user documentation. If that's not feasible we should change the docstrings currently in the testing code that describes a perfect sync. Note that the documentation PR is not merged at this time of writing.

[pmatulis]

In a new PR the user-facing payload upgrade documentation was altered to put nova-compute immediately after nova-cloud-controller.