Utility to migrate through installed version application upgrades

[nfx]

Sometimes, upgrading an installation requires changes not only to config format, but also application state in the form of data assets and workspace assets, like cluster policies, cluster or dashboard configurations, which results in outages while rolling new versions out.

We need:

• in-wheel package name to iterate over submodules
• every submodule to be named as v0.12.5.py (verify if this works)
• every submodule providing upgrade(WorkspaceClient) function to deploy this version upgrade
• current app version determined from ~/.app/version.json file created by the WheelsV2.upload() method
• figure out if we execute version migrations atomically or just rely on a version diff and have the upgrader code be resilient to re-applies, possibly via exception ignores and multiple files per release

[nfx]

Fixed in #50