Build K8s 1.19.x and 1.20.x boxes using Shepard

[sabbyanandan]

As part of the K8s stack validation, it'd be good to build and run our ATs on 1.19.x and later versions of K8s clusters.

Note: K8s 1.17.x has been retried upstream and it will be removed from the supported versions.

[sabbyanandan]

@ilayaperumalg @jvalkeal: As part of the enterprise readiness checklist, we have to use validate our offering on Shepard. Let's give this another try. Maybe when we are at it, let's also make sure the underlying TKGm versions that Shepard uses point to K8s 1.19 and 1.20, at the least.