Recommendation around master node sizing to handle upgrades
Recommendation around master node sizing to handle upgrades

This commit:
- Adds a recommendation around master node sizing to handle upgrades and
  failures on large and dense clusters as the resource usage is expected
  to increase in such situations.
- Adds a note that the data points are only valid for OpenShift clusters
  using OpenShiftSDN as the network plugin.

vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs