Add check to not push an empty config

[Cybertinus]

When the process of generating new configuration is done, Kees should not only check if the syntax for the vendor in question is correct, but it should also check if the new config actually does something. A config without any BGP peers, or no OSPF enabled interfaces can have a correct syntax, but the network will go down. This should also be checked.
When this check is added, it should also run in the Travis validation, when this is configured too.