add allowPrivilegedContainers to flavors

[dguendisch]

What this PR does / why we need it:
Adds allowPrivilegedContainers as an additional option in the flavor configuration. Gardener shoots default to true is this option is not set, hence you can use it to disable privileged containers for certain tests or have your tests run against both: shoots with allowPrivilegedContainers="true" and shoots with allowPrivilegedContainers="false"

Release note:

`allowPrivilegedContainers` can now be customized in the flavor configuration.

[schrodit]

Do we maybe want to add it as metadata field so that we can see the differences in the execution?
And we could also think of adding it to our dimensions to reflect it in the dashboard/alerting?
WDYT?

https://github.com/gardener/test-infra/blob/master/pkg/testmachinery/metadata/types.go#L18

[dguendisch]

Regarding metadata, I though it would be enough to distinguish via flavor description, but you are right, a dedicated metadata is better, I'll add that, thanks for the suggestion.

[schrodit]

just a minor nit rest is lgtm

[dguendisch]

added metadata support, hope I didn't miss some places, ptal

[schrodit]

we also need to read the metadta from the testrun here https://github.com/gardener/test-infra/blob/master/pkg/testmachinery/metadata/metadata.go#L57

How about adding it also here https://github.com/gardener/test-infra/blob/master/pkg/testmachinery/metadata/metadata.go#L41 so that it will also be displayed in the tm dashboard and the slack notifications?

[dguendisch]

we also need to read the metadta from the testrun here https://github.com/gardener/test-infra/blob/master/pkg/testmachinery/metadata/metadata.go#L57

Added.

How about adding it also here https://github.com/gardener/test-infra/blob/master/pkg/testmachinery/metadata/metadata.go#L41 so that it will also be displayed in the tm dashboard and the slack notifications?

I actually envisioned to put it in the description of the flavor configuration, but sure, putting it directly in the code is more consistent in the end, so added it.

PTAL

[schrodit]

just one minor nit. rest is lgtm