[k8s] validation: kube-conformance s/v1.11/v1.15/

[k8s] validation: kube-conformance s/v1.11/v1.15/

Bump kube-conformance from v1.11 to v1.15 to pick up newer test images,
e.g. nginx-slim only has arm64 support starting with the next version
after the one pinned by e2e tests in kubernetes v1.11.10, while
e2e.test binary only has proper etcd arm64 support starting with 1.15.

- update README;
- bump dockerhub image tag;
- bump git submodule pinned commit from upstream
  heptio/kube-conformance;
- bump tag in image referenced by sonobuoy configuration;

While at it, also align related resource versions in K8s image:
- bump kubectl/gingo/e2e.test in K8s image from v1.13.0 to v1.15.x
  (latest minor release available for the 1.15 major release);
- pin sonobuoy binary version to v0.14.3 tag;
- bump sonobuoy timeouts to accomodate runs on slower hardware;

Change-Id: Ia0e632219694a63e9cdf06d7e6dad89cff016361
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>