gateways: clean up, fix and improve config

[Noki]

strom-gw is the only gw with a network file in group vars and only OpenWRT antennas are listed there as assignments but not the other antennas: https://github.com/freifunk-berlin/bbb-configs/blob/master/group_vars/location_strom/networks.yml

In addition strom-so-5ghz is the only remaining OpenWRT device, the others are gone.

We should clean up the config and maybe also migrate all gateways to single file format, add SNMP monitoring and DFS resets.

• migrate all gateways to single file format - PR gateways: unify configs #713
• add SNMP monitoring for saarbruecker Treewide: Add snmp monitoring for some switches #760
• add SNMP monitoring for ohlauer Treewide: Add snmp monitoring for some switches #760
• add DFS resets for all gateways

[Noki]

This is related to #760 and SNMP monitoring can be checked of the list once that PR is merged and flashed to all gateways.