delete missing nodegroups based on config file

[errordeveloper]

This would improve upgrade docs.

[errordeveloper]

With respect to the docs, right now this is what we suggest to the users

# collect old nodegroup names
old_nodegroups="$(eksctl get ng --cluster=<clusterName> --output=json | jq -r '.[].Name')"
# create new nodegroups based on config file
eksctl create nodegroup --config-file=<path> 
# delete old ones
for ng in $old_nodegroups ; do eksctl delete nodegroup --cluster=<clusterName> --name=$ng ; done

so the idea is to get rid of jq and the for loop

eksctl create nodegroup --config-file=<path> 
eksctl delete nodegroup --config-file=<path>  --only-missing

[errordeveloper]

After some refactoring (#665, #670 & #671), we are ready to add --config-file and --only, so that eksctl delete ng matches eksctl create ng, and then we can add --only-missing as well.

[errordeveloper]

See also #682.