Allow Expanding ClusterNetworks

Goal: Allow the expansion of ClusterNetworks.

Problem:
In OpenShift 3.x, we had a procedure for expanding the ClusterCIDR of an existing cluster without disrupting running workloads. It was, however, manual and error prone. Now that we have the cluster network operator, we should once again allow this operation in an improved manner.

Why is this important?
Clusters that grow organically may run out of pod or service IP space. There are safe ways to expand their CIDR space.

Dependencies (internal and external):

Prioritized epics + deliverables (in scope / not in scope):

Not in scope:

• Renumbering nodes, especially master nodes
• Any CIDR modifications other than expansion

Affected packages or components:

• cluster-network-operator
• openshift-sdn
• ovn-kubernetes
• openshift-kube-apiserver-operator

Estimate (XS, S, M, L, XL, XXL):

Previous Work:

Open questions:

Acceptance criteria:

Epic Done Checklist:

• CI - CI Job & Automated tests: <link to CI Job & automated tests>
• Release Enablement: <link to Feature Enablement Presentation> 
• DEV - Upstream code and tests merged: <link to meaningful PR orf GitHub Issue>
• DEV - Upstream documentation merged: <link to meaningful PR or GitHub Issue>
• DEV - Downstream build attached to advisory: <link to errata>
• QE - Test plans in Polarion: <link or reference to Polarion>
• QE - Automated tests merged: <link or reference to automated tests>
• DOC - Downstream documentation merged: <link to meaningful PR>
• Notes for Done Checklist: 
  • Adding links to the above checklist with multiple teams contributing; select a meaningful reference for this Epic.
  • Checklist added to each Epic in the description, to be filled out as phases are completed - tracking progress towards “Done” for the Epic.