Consistent preflight validations for all tasks

[KnVerey]

Follow-up to #337

The deploy task already checks whether it can reach the API server before proceeding, so that it can fail with a relevant message instead of failing whichever other request happens to be made first. We should do the same in the runner task. In fact, we should make sure that all globally applicable preflight validations are done consistently by all four of our tasks. One possible approach would be to introduce a config validator object for this purpose, as prototyped here: https://github.com/Shopify/kubernetes-deploy/compare/config_validator.

• Determine all of the appropriate pre-flight validations should be in place across all tasks
• Make sure they are being executed consistently (including order)

Considerations

• Maybe we make a generic validation class that can be reused or something specific for each task

[dturn]

#533 Added a TaskConfig and TaskConfigValidator to each task. But it only added the full set of pre-flight validations to the RestartTask. We need to add it to the remaining tasks to close out this issue.

When working on adding the full set of pre-flight validations to the remaining tasks, consider if we need sub classes like DeployTaskConfig and DeployTaskConfigValidator you can take inspiration from https://github.com/Shopify/kubernetes-deploy/compare/config_validator (or punt this to a new issue).

[RyanBrushett]

We should look into how we report warning/errors as part of this too.

See this comment for why I mention this. Thanks @dirceu !

[dturn]

Done.