The repo is Python 3 now, so update hacking to version 3.0 which
supports Python 3.
Fix problems found.
Update local hacking checks for new flake8.
Remove hacking and friends from lower-constraints, those are not needed
for co-installing.
Change-Id: I926efaef501f190e78da9cab40c1e94203277258
Add flavor_id as an option during cluster create. If not given,
the default is taken from the cluster template.
Add flavor_id in the Cluster object and use that instead
of the one from ClusterTemplate.
Update both magnum and magnum cli documentation to reflect the above changes.
Partial-Bug: #1699245
Change-Id: Ib60c05cce1cf2639ca4740abdd264403033433f9