Add network connectivity command and default PodNetworkConnectivityCheck namespace

[cptmorgan-rh]

• Add omc network connectivity to summarize PodNetworkConnectivityCheck data from pod_network_connectivity_check/podnetworkconnectivitychecks.yaml
• Default omc get podnetworkconnectivitychecks to openshift-network-diagnostics when namespace is unset
• Document the commands in README.md

Example output:

$ omc network connectivity
NAME                                                        TARGET                    REACHABLE   LAST_FAILURE_REASON   LAST_FAILURE_MESSAGE
network-check-source-apiserver                              https://10.0.0.1:6443     True
network-check-source-to-load-balancer-api-internal          10.10.10.1:8080           True        TCPConnectError       network-check-source-to-load-balancer-api-internal: failed to establish a TCP connection ...