Remove 'cinder-manage config convert'

Remove 'cinder-manage config convert'

This convers the old flagfile style config files into the the
new .ini style config files. Since Cinder is new and it's first
release will only support .ini style files there's no need for
such a command.

Change-Id: Ie2f4cd8b6f5fc66547a59d39aa3d5eb896b1ddf4
Reviewed-on: https://review.openstack.org/9871
Reviewed-by: Huang Zhiteng <zhiteng.huang@intel.com>
Reviewed-by: Vish Ishaya <vishvananda@gmail.com>
Approved: John Griffith <john.griffith@solidfire.com>
Tested-by: Jenkins