Show unavailable sliver ifaces with the cause (#216).
Show ifaces forbidden reason on batch sliver creation.
Use node resources data to determine ifaces availability.
Tests #216: create interfaces is_allowed unittests.
Isolated iface requires direct interface on the node.
Isolated iface cannot be added on bulk creation.

Parent iface should be choosed on isolated iface creation
but is not possible on bulk creation, thats why is disallowed.
Minor changes to disabled iface types in sliver bulk creation (#216).
Merge branch 'feature216'