Description of problem:

In both RHOSP7 and RHOSP8, the function IpNetnsCommand.exists in neutron/agent/linux/ip_lib.py cannot correctly parse "ip netns" output.

Following is an example. Given the current parsing method, the (id: x) won't be stripped.
[root@overcloud-controller-2 neutron]# ip netns
qdhcp-511e26fd-ca31-4521-adba-f328d7777c1b (id: 7)
qrouter-901ab96c-7375-4cf0-9974-852d2fdbe341 (id: 6)
qdhcp-d8f491c2-b102-4ef9-8687-aae36b04a3d8 (id: 5)
qdhcp-8a1f6447-1444-41e9-bda6-ecc0d5f5607e (id: 4)
qrouter-75612b50-f359-4a20-abfe-2f2718e8e688 (id: 3)
qdhcp-702d8ba9-5baa-4df4-8873-875714fecc38 (id: 2)
qdhcp-94f53458-17ca-4b2f-9ba2-7c4b801a8d6d (id: 1)
qdhcp-1a1e4582-3c42-40e3-a667-71b08937c486 (id: 0)

The impact of this is that l3-agent will always try to create a router name space even when the name space exists.

A proper implementation of that function is at
https://github.com/openstack/neutron/blob/stable/kilo/neutron/agent/linux/ip_lib.py#L584-L591

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