[jump] Add simple check for required Linux bridges

[jump] Add simple check for required Linux bridges

- MaaS requires PXE/admin to be a Linux bridge;
- if virtual nodes are present, they should be hooked to a proper
  Linux bridge for the Public network, but only throw a warning if
  not (and create a mock public virsh network instead);
- if both virtual and baremetal nodes are present, Public bridge is
  indirectly mandatory (we can't mock it);

JIRA: FUEL-339

Change-Id: Idfe99d66c49eadc56cb3d94ca4db3467fb76d388
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>