Add ability for empty strings to be used as override, refactor clusterConfig

[George-Aeillo]

In this PR, I added the ability to pass in empty strings as overrides to a cluster configuration. This PR also isolates the config structure to only the initial setup of a command, while the actual logic itself only uses the cluster configuration. To accomplish this, the fields in the cluster configuration and config are now pointers to scalar types, rather than the scalar types themselves. These changes were tested on starting and deleting libvirt clusters, CAPI (self managed and non self-managed), image upload, image create, cluster template, and the cluster join command through an in-place upgrade from OCNE 1.7 to 2.0 in an air-gapped environment. I have attached the associated yaml files illustrating how an empty string can be passed through

default.yaml

name: defaultCluster
registry: private-registry.com
...

testConfig.yaml

name: clusterToOverride
registry: ""
...

cluster configuration

name: clusterToOverride
registry:  ""
...