Make JDBCScheme table alias optional to allow usage on Vertica database 

We at Etsy have started using a Vertica database as a source for big data jobs. Vertica does not allow connections to a particular database/schema. This implies that table names have to be prefixed with the schema name like "schema_name.table_name". This causes JDBCScheme to fail as Vertica, like many other databases, does not like a dot in the table alias. This changeset makes the table alias optional allowing JDBCTap to be used with Vertica.