Create vlan bridge validation on netgear ar71xx

[sarcasticadmin]

Description

Was unable to ping mgmt interface from anything other than the default gateway with the following configuration on the management interface for the AP:

config switch_vlan                                                                                                                                                          
        option device 'rtl8366s'                                                                                                                                            
        option ports '0t 3t 5t'                                                                                                                                             
        option vlan '44'
config switch_vlan                                                                                                                                                          
        option device 'rtl8366s'                                                                                                                                            
        option ports '0t 3t 5t'                                                                                                                                             
        option vlan '503'
...
config interface 'mgmt'                                                                                                                                                     
        option ifname 'eth0.44 eth0.503 eth1.44 eth1.503'                                                                                                                                     
        option proto 'dhcp'                                                                                                                                                 
        option reqopts '224 225'                                                                                                                                            
        option type 'bridge'

NOTE: It seems to give me weird behavior at times when mixing vlans

I think it might be a good idea to do a little bit of validation on switch_vlan config for netgear ar71xx to confirm that all interfaces in a bridge do in fact exist. But whats also not clear is that whether or not its ok to mix vlans like above. We have had some success in doing this in the past years but Im not sure if theres some kind of regression.

Acceptance Criteria

• Tests/Validation being added to template output for netgear ar71xx network config
• See if mixing vlans and interfaces is OK in current builds

[sarcasticadmin]

Scenario could be fixed via #367 but will remain open since could be outstanding even after upgrade