Support more machine image version patterns in flavors

[dguendisch]

In flavors we can today specify either a fixed machine image version or latest.
For certain tests we want to test all non-deprecated versions of a certain machine image.
It would be helpful if the flavors would support such a version specification similar to how it was done for the k8sVersions (were we support latest and semver).

Example:

      machine:
        type: n1-standard-2
        image:
          name: gardenlinux
          version: "*"
          filterDeprecatedVersions: true