[19.04] openstack-upgrade action should allow re-runs

[I think this bug affects all the OpenStack charms, but I'm going to leave this bug initially assigned to charm-keystone for you to decide if link it to all the projects]

"openstack-upgrade" action snippet:
"""
    if (do_action_openstack_upgrade('swift',
                                    do_openstack_upgrade,
                                    CONFIGS)):
        config_changed()
"""

In general, all charms check a relevant package (nova-common, neutron-common, swift, etc) and starts the upgrade procedure if such package is not up to date in comparison to the archive listed in "openstack-origin" config parameter.

However, previous executions with errors would leave an environment unconfigured (maybe even needing to manually resolve an APT conflict) and future re-runs of the action would skip the process because said "chosen" package is already up to date.

Would it be possible to change that snippet to something like (always run):
"""
    do_openstack_upgrade(configs=CONFIGS)
    config_changed()
"""

By doing so, already upgraded packages will be skipped, but extra configuration files would be regenerated.

FWIW, I've had to downgrade that "chosen" package in a unit (and use the proposed snippet in the action in a few others) so as to allow the action to be successfully run.