create_schema if auto-generate-sql also set.

mysqlslap uses a schema to run its tests on and later
drops it if auto-generate-sql is used. This can be a
problem, if the schema is an already existing one.

If create-schema is used with auto-generate-sql option,
mysqlslap while performing the cleanup, drops the specified
database.

Fixed by introducing an option --no-drop, which, if used,
will prevent the dropping of schema at the end of the test.