history: made several changes to vm disks tables

- dropped old indexes on image_id and created indexes on
  vm_disk_id.
- changed image_id to nullable and vm_disk_id to not nullable.
- changed vm_disks_usage_daily_history time column type to date.
- changed disk description to 500 chars

Change-Id: Ib4f01238d50a8bd21b4b08f5aa03b2c6c771df93
Signed-off-by: Yaniv Dary <ydary@redhat.com>