Fix libvirt network name conflict

Management network defined around lines 36-41 is using 192.168.145.0/24
network address, and then 'default' as the network name.
Libvirt (on Fedora 20 at least) is using 192.168.100.0/24 network with
the same name, which causes OMV to fail during "vagrant up".
"virsh net-destroy/net-undefine default" is not a solution, since
one might actually use 'default' network already.
Proper fix is to either use network2 = IPAddr.new '192.168.100.0/24',
or use own name for the mgmt network.
This patch implements the 2nd solution.