Switched snapshot owner to AccountNumber from TenantId.

This PR switched the Owner of Snapshots from the TenantId to the accountNumber. I believe that this is causing FB 4795. Amazon uses the accountNumber for this value, and DCM compares this value to the accountNumber in its cloud_account table to determine whether a snapshot is owned by a user (or has been shared from another user).

Looking back at previous versions of dasein-cloud-openstack (like 2013.02) it seems that the accountNumber was used as the owner in the past, so I'm not sure why this was changed. Was there a particular reason for that?