Remove broken config_drive image_href support.

image_href support has not been working since at least shortly before
Folsom release.  This is a good indication that this functionality is not
used.  As far as I can tell, the docs also do not match what was
supported.  An image ID was required, but docs show examples with full
hrefs.

DocImpact

http://docs.openstack.org/developer/nova/api_ext/ext_config_drive.html

References to supporting image_hrefs should be removed.

This patch also removes the hack that passed a config_drive_id via
the Instance dictionary when config_drive_id is not a valid Column for
Instance.

Fixes bug 1186401

Change-Id: Iced7bc8e278cb9f208183f1dbb7a293675a47eae
Merge "Remove broken config_drive image_href support."