Fixes #6444 - add support for virtual NICs

* user/pw are for BMC only but accessibility should be defined in STI base, otherwise you'll have hard times e.g. when you generate generic form for interface and it turns out to be BMC, not sure about validations for these, as they can be blank or any string?
* physical device and identifier shouldn't be restricted until we decide whether foreman always know all interfaces, I think we should support having eth0.1 with physical device eth0 even if eth0 does not exist (yet?), also I'm not sure what different kind of interfaces we are going to support and what their names can be, so identifier can be any string or nil (backward compatibility)
* regarding virtual and link booleans, I'm not sure it's a good idea to enforce boolean value there, they are casted automatically by rails which parses '0' and '1', we could use inclusion validator with [true, false] but I don't see big benefit from doing this