Improvement: check for duplicates

[Noki]

It's easy to make careless mistakes, e.g. when copying a configuration. We should therefore implement checks that check for duplicates and throw an error if there are any.

• prefix must be globally unique in all vlans across all networks.yml files
• ipv6_prefix must be globally unique across all networks.yml files
• ipv6_subprefix must be unique within a networks.yml file
• name of a vlan must be unique within a networks.yml file