Move presubmit jobs to AWS CI

[Jeffwan]

Community is asking different WG to own their infra and community won't provide a common shared testing infra anymore. Sees kubeflow/testing#752 for more details.

Pytorch migration works well and here's the PR to update test case to work on AWS Prow+ Argo cluster. kubeflow/pytorch-operator#305

However, current mxnet doesn't really have any jobs defined. https://github.com/kubeflow/mxnet-operator/blob/master/prow_config.yaml

whoever can contribute an e2e tests, we can follow what pytorch operator did and kick it off in new testing infra