Fix db_version failed with wrong arguments

The `keystone-manage db_version` command was failing with an error
like

 TypeError: db_version() takes exactly 2 arguments (1 given)

This is because an engine wasn't being passed to db_version().

Change-Id: I20b4cb10f9c83fc672de2838bac8236938b92e32
Closes-Bug: #1284341
Merge "Fix db_version failed with wrong arguments"