Bug fix #318 Kimchi fails creating new network

NetworkModel does not have the method _get_network. So, the method
get_network form NetworksModel become a static method and now are
available to be use in src/kimchi/model/networks.py.

Signed-off-by: Ramon Medeiros <ramonn@linux.vnet.ibm.com>
Add one more case to network tests

Add a test without subnet while creating a network interface, to test
bug https://github.com/kimchi-project/kimchi/issues/318.

Signed-off-by: Ramon Medeiros <ramonn@linux.vnet.ibm.com>