Normalized fields policy

The issue of which ES fields should be normalized across all plugins and the rationale for the policy should be documented and verified.

As we get more usage of Searchlight and more plugins written, this keeps coming up [1], [2].

We should document the policy in multiple places:
- Searchlight API [3]
- Searchlight Plugin documentation [4]
- Authoring Searchlight Plugins [5]
- OpenStack API Ref for Searchlight [6]

We should also expand the list of normalized fields to verify (i.e. the "fields" list in unit/test_plugin.py::verify_normalized_fields)

[1] https://bugs.launchpad.net/searchlight/+bug/1496461
[2] https://bugs.launchpad.net/searchlight/+bug/1545069
[3] http://docs.openstack.org/developer/searchlight/searchlightapi.html
[4] http://docs.openstack.org/developer/searchlight/plugins.html
[5] http://docs.openstack.org/developer/searchlight/authoring-plugins.html
[6] http://docs.openstack.org/developer/searchlight/authoring-plugins.html