Fix type of snapshot_id column to match db
Fix type of snapshot_id column to match db

 * Migrations create a snaphsot_id column that is an integer, but
   models incorrectly list it as a string.
 * The above issue causes errors in certain situations when trying
   to list instances. It seems to be only certain db configurations
   or versions of sqlalchemy that exhibit the issue.
 * Issue was fixed in trunk as part of a conversion to uuids in
   commit 407e16b863bac1dfbf4e954837009abf9c17f018 so a straight
   backport is not possible
 * Fixes bug 962615

Change-Id: I8e9fc9712c141822890feffee838faf8b41ee4f5

yrobla/nova,yrobla/nova,yrobla/nova