VPNaaS Device Driver for Cisco CSR

This is the device driver for the vendor specific VPNaaS plugin. This
change relies on the service driver code (review 74144), which is also
out for review.

Note: Support for sharing of IKE/IPSec policies (which is currently
      prevented by the service driver code), will be done as a later
      enhancement.
Note: Needs Tempest tests updated/created to test this.
Note: To run, this needs an out-of-band Cisco CSR installed and
      configured.
Note: This uses a newer version of requests library and a new httmock
      library. Until these are approved (75296), the UT will be
      renamed to prevent testing the REST client API to the CSR.

Change-Id: I4f73f7fa1bfcdc89a35ffe63dd253f8eede98485
Paritally-Implements: blueprint vpnaas-cisco-driver
Merge "VPNaaS Device Driver for Cisco CSR"