Extension health checks support Progressing status

[rfranzke]

What this PR does / why we need it:
With this PR the extension health check library is enhanced to allow individual health checks to return the Progressing status. This is helpful in order to provide more accurate status information and less false negative reports.

Also, the worker extension health check particularly was improved. It can now better detect regular scale-down and scale-up situations. Consequently, the "too many nodes" check was removed from Gardener's generic health check and moved into the responsibility of the worker extension health check (as it can more accurately determine what is going on).

Release note:

The extension health check library does now allow individual health checks to return the `Progressing` status. This allows to provide more accurate status information and less false negative health reports.

[rfranzke]

/kind/enhancement

[danielfoehrKn]

Code looks good to me - nice to have now error codes & progressing condition status allowing more sophisticated health checks in general.
Have you already build an image that I can re-use for testing it with the extension providers?

[rfranzke]

Have you already build an image that I can re-use for testing it with the extension providers?

No, I haven't built an image, I was revendoring the AWS provider, started it locally on my machine and then performed my tests (easier/faster turn-around cycle).

[danielfoehrKn]

Tested and works for me. Only nits regarding logging.

Maybe as a special remark for reviewers you could add next time that you have to add the

			scheme.AddKnownTypes(machinev1alpha1.SchemeGroupVersion,
				&machinev1alpha1.MachineDeploymentList{},
				&machinev1alpha1.MachineSetList{},
				&machinev1alpha1.MachineList{},
			)

to the scheme in the extension provider- in case reviewers are not aware.

[danielfoehrKn]

Build is failing

[rfranzke]

/needs/review

[timebertt]

Looks quite good, I have only minor comments for you :)

[timebertt]

Looks good now, only the loop thingy left :)

[timebertt]

Nice, thanks for incorporating my suggestions.
/lgtm now

[rfranzke]

@danielfoehrKn any more feedback?