Add documentation so the API spec generator can do something
for the VLAN setting.  Also enforce validation of the ingress
and egress priority maps; 802.1D priorities are limited to
values from 0 - 7 and Linux SKB values are 32 bits.

Also, the 'slave' property is a duplicate of the 'master'
property added to NMSettingConnection in the bonding work
so we can get rid of it and use 'master' instead.