Look into removing the public_vlan var

[akrzos]

It is not clear to me this is in use anymore and is a rather confusing var. We should seek to remove it to avoid confusion.

[dbutenhof]

@akrzos You apparently started this in #485 but gave up, suggesting this is more complicated than it might seem at first glance. Can you share more context?

[akrzos]

I didn't have the means to test it. The public_vlan var is confusing and could actually be entirely done away with if we made SNOs deploy on the private network by default, such that their networking matched that of MNO/BM clusters. Also this matching would make it easier to maintain. If we deem it necessary, they we should have a var for use_public_lab_dhcp and that would place the SNO on the public lab's dhcp network instead (This is the default today).