Add support for configuring extra bridges

[seumasdunlop]

We have a requirement to pass multiple network interfaces through to the kubernetes nodes. I've implemented this by adding a list of extraBridges for each instance that is used to create additional network_interface sections in the libvirt_domain and populate the cloud_init_network_static.tpl file to configure the new network interfaces in the VM.

If you're happy to proceed I'll create a pull request. Can any of the issues I've created today be grouped into a single pull request or do you want one for each issue?

[MusicDin]

Hi,

thanks again for opening all these issues.
If you could open separate pull requests, I would be very grateful as we can then discuss each issue separately further.