Flush tokens in batches with DB2

When there is a very large number of expired tokens to be flushed,
keystone-manage token_flush will fail with DB2 because it fills
the transaction log. This fix causes tokens to be flushed with DB2 in
batches of 100 at a time to prevent the transaction log from
filling up.

Closes-Bug: #1257323
Change-Id: Ic57d8795a995462fc277ec9655eca9b460ffcbee
Merge "Flush tokens in batches with DB2"