Use minikube in Ubuntu based tests

[kashifest]

Recently we have seen that CentOS E2Es are failing in different tests since the CI infra shift. In contrast, ubuntu e2es are rather stable. CentOS failures are mostly causing problems and main branch PRs which are left hanging for unrelated issues. There are few steps which we can perform the phase out CentOS tests for the time being:

• Use minikube in Ubuntu to make sure ironic is deployed as k8s deployment instead of local containers as it is now. This is the fundamental difference currently between Ubuntu and CentOS tests - Make minikube the default ephemeral cluster #1404
• Phase out CentOS tests as required from prow in project-infra repo as well github branch protection settings.
• Bring back CentOS (if deemed necessary) once the CI infra has better hardware and tests are not flaky. CentOS periodics which are not being phased out can signal this.

[kashifest]

/triage accepted