FEATURE: Create a volume from a snapshot at different volume av.zone.

Version 2 for openStack Folsom release:
- nova/db: Added new 'snapshot_get_host' and 'snapshot_get_all_by_host' API calls.
- nova/volume: Export all snapshot as iSCSI targets at Start of nova-volume
- nova/volume: Export snapshot as iSCSI targets at snapschot create
- nova/volume: Remove snapshot iSCSI target at snapschot delete
- nova/volume: Use iSCSI snapshot target as source for volume creating, filtered by 'iscsi_ip_filter' parameter.
- Do not use iscsi discovery if we know host name.