LXC appliance; bridged networking - some VM envs require network "promiscuous mode"

[JedMeister]

After tons of struggles trying to get bridged networking working on our LXC appliance, Sergio discovered that some VM environments require "promiscuous mode" to be enabled in the network settings (i.e. within the config of the VM guest).

At the very least we should note that somewhere within the LXC appliance docs. Perhaps we can even add it to the OVA build config; so it's preconfigured for VMs?

[JedMeister]

As per #1520 LXC has been deferred for now...