[baremetal] Move VCP iface config to own yml

[baremetal] Move VCP iface config to own yml

- move linux.network.interface definitions to their own classes, which
  also removes the previous requirement of defining {dhcp,single}_nic
  parameters in classes that don't actually use them;
- drop now useless {dhcp,single}_nic parameters;
- expand linux_{dhcp,single}_interface macros, since they cause issues
  with reclass dict-merge while attempting to override sub-vars (i.e.
  it's not possible to set 'enabled: false' via reclass interpolation);

JIRA: FUEL-310

Change-Id: I29d921f545e761de335a60e242a4523d13b06c78
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>