Support Magnum upgrade API

[jkmw]

Currently there is no support for the cluster upgrade-action. This PR implements the missing api-call.

For #2031

API doc:

• https://docs.openstack.org/api-ref/container-infrastructure-management/?expanded=upgrade-a-cluster-detail#upgrade-a-cluster

source from cluster-actions controller:

• https://github.com/openstack/magnum/blob/e8467e94d88b6feb67ff7c167f5e167cd452c772/magnum/api/controllers/v1/cluster_actions.py#L58
• https://github.com/openstack/magnum/blob/e8467e94d88b6feb67ff7c167f5e167cd452c772/magnum/api/controllers/v1/cluster_actions.py#L132

[coveralls]

Coverage increased (+0.01%) to 79.566% when pulling ec5167e on jkmw:magnum-clusters-upgrade into c137dd5 on gophercloud:master.

[coveralls]

Coverage increased (+0.01%) to 79.566% when pulling ec5167e on jkmw:magnum-clusters-upgrade into c137dd5 on gophercloud:master.

[theopenlab-ci]

Build succeeded.

• gophercloud-unittest : SUCCESS in 4m 01s
• gophercloud-acceptance-test : SUCCESS in 1h 47m 22s
• gophercloud-acceptance-test-ironic : SUCCESS in 29m 09s

[jtopjian]

LGTM