Add APIv3 support to make host optional on evacuate.

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: I907ea4ef39cfad50a48cb80bceba217fdf482f6e
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>
Merge "Add APIv3 support to make host optional on evacuate."