support create subnet with same network name in different project

[Yi Zhao]

if there are two network with same name in different project, openstack client can not create subnet with this network name.

for example,
project "admin" has network "test-net1"
project "demo" has network "test-net1"

when user create a subnet in the project demo with the network name "test-net1", it reports error: "More than one Network exists with the name 'test-net1".

$ openstack subnet create --project demo --network test-net1 --subnet-range 11.0.0.0/24 subnet1
More than one Network exists with the name 'test-net1'.

The problem is due to there lacks "project" filter when find the networks.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/476414