ci: enable K8s 1.20.0 to run the integration tests
ci: enable K8s 1.20.0 to run the integration tests

The integration tests need to run on all the supported version of K8s.
With the release of 1.20.0, we now enable the tests there. To keep the
tests running in master on 5 versions, we drop 1.17 and keep 1.15 so
we can keep testing against v1beta1 extensions in master.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

rook/rook,rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook,rook/nfs,rook/nfs