model upgrades only increment 1 major version at a time

[lathiat]

When upgrading a model (both controller and user models) the "juju upgrade-model" command will only increment one major version at a time.

e.g. from 2.4.10 -> 2.6.10 -> 2.7.6

While this is not surprising, this is not currently documented.

It should be document as it impacts upgrade planning in terms of time taken and also impact on nodes as config-changed etc hooks on large openstack environments take some time to execute and will extend maintenance windows.