Add APIv2 support to make host optional on evacuate

In the event of an unrecoverable hardware failure, support needs to
relocate an instance to another compute so it can be rebuilt.

The changes involved in this patch are:
  [*] Add a new v2 extension to determine that the host argument
      on evacuate is now optional.(Extended_evacuate_find_host)
  [*] Doc regeneration.

DocImpact: The evacuate target host is now optional.
If 'host' field is not sent in the request, the scheduler will
determine the target host.
This will include nova client changes ( on the proper commit ) to support
this new optional parameter.

Implements: blueprint find-host-and-evacuate-instance

Change-Id: Ib34fb3120263b746ad2f8fe89c14137e36a07a53
Co-Authored-By: Juan M. Olle <juan.m.olle@intel.com>
Co-Authored-By: Andres Buraschi <andres.buraschi@intel.com>
Co-Authored-By: Anuj Mathur <anujm@thoughtworks.com>
Co-Authored-By: Navneet Kumar <navneetk@thoughtworks.com>
Co-Authored-By: Claxton Correya <claxton@gmail.com>
Gate on F402/pep8

rename instances of loop variables to prevent them from shadowing
imports. This strict check was added in hacking 0.9

Change-Id: Ib5e162b85e3c6931f213731c3febd3dd8ba0b4b1
Merge "Add APIv2 support to make host optional on evacuate"
Merge "Gate on F402/pep8"