Add requests_mock to test-requirements.txt
Add requests_mock to test-requirements.txt

Already in global-requirements. Will use for VPN unit tests
(bug 116018).

Change-Id: I140caebd3bc4f4cbf9563698bf0143f68456c762
Closes-Bug: 1358472

citrix-openstack-build/neutron-vpnaas,openstack/neutron-vpnaas,openstack/neutron-vpnaas,dougwig/x-neutron-vpnaas