stolonctl spec: don't show some null clusterspec options
stolonctl spec: don't show some null clusterspec options

* show only changed options and omit all the empty/null options
* when using --defaults show ALL the options

Do this by using two copies of the clusterspec struct, one with all json tags
with omitempty and one without omitempty that will be casted from the original
cluster spec (so we will get an error when we forgot to updated them when
changing the ClusterSpec struct)

sgotti/stolon,sorintlab/stolon,sgotti/stolon,sorintlab/stolon