Add networks purge command to cleanup job

[Rajalakshmi-Girish]

Stale networks are sometimes left behind in the workspace. Adding this would make sure the networks are also cleaned up thoroughly.

Also, about to create an IBM support ticket for networks that are throwing Internal server errors while deletion.

[ppc64le-cloud-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mkumatag, Rajalakshmi-Girish

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[config/OWNERS](https://github.com/ppc64le-cloud/test-infra/blob/master/config/OWNERS)~~ [Rajalakshmi-Girish,mkumatag] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ppc64le-cloud-bot]

@Rajalakshmi-Girish: Updated the job-config configmap in namespace prow at cluster default using the following files:

• key cleanup-k8s-periodics.yaml using file config/jobs/periodic/housekeeping/cleanup-k8s-periodics.yaml

In response to [this](https://github.com/ppc64le-cloud/test-infra/pull/468): >Stale networks are sometimes left behind in the workspace. >Adding this would make sure the networks are also cleaned up thoroughly. > >Also, about to create an IBM support ticket for networks that are throwing Internal server errors while deletion. > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.