Prevent assigning IP address from vm_network pool to mgmt interface.

In provisioning process new node get IP address equal to last assigned
address + 1. Before get last assigned IP we must filter out addresses
from vm_network pool to not choose one of it as a base for
calculation of IP address reservation for mgmt interface of new node.
Merge pull request #11 from michalskalski/master

Prevent assigning IP address from vm_network pool to mgmt interface.