Move rebuild to conductor and add find host logic

[*] Move rebuild instance to conductor task api will help
    in consolidating the logic for both evacuate instance and
    rebuild instance. Rebuild instance and evacuate instance
    uses the conductor task api for rebuild instance workflow.
[*] Add logic required to support rebuild without host.
    (used by evacuate)
[*] Refactor rebuild action to pass the instance host as argument,
    instead of None.

Evacuate and Rebuild will behave the same way as today.
Modifications for the new feature in API will be implemented in
new patches.

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

Change-Id: I4b1a9c2227574db7179441d531a49145f213ea87
Co-Authored-By: Juan M. Olle <juan.m.olle@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 "Move rebuild to conductor and add find host logic"