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

openstack/networking-odl,CiscoSystems/networking-odl,schoksey/networking-odl,openstack/networking-odl,CiscoSystems/networking-odl,FedericoRessi/networking-odl,flavio-fernandes/networking-odl,mestery/x-stackforge-odl,flavio-fernandes/networking-odl,schoksey/networking-odl,openstack/networking-odl,FedericoRessi/networking-odl