List arguments should use proper nargs in argparse

[PackeTsar]

Currently, the command meraki -ddd organizations createOrganizationNetwork --organizationId xxxxxxxxx --name "Meraki-CLI Network" --productTypes appliance switch will only pass the first --productTypes argument value (appliance) instead of accepting multiple.

When building the argument, we should use something like nargs='+' to automatically create the list expected by the function.