Fix database quoting problem in collector 'info_schema.tables'

[oblitorum]

In collector/info_schema_tables.go, the SQL it uses to retrieves the table meta data is

SELECT
	TABLE_SCHEMA,
	...
FROM information_schema.tables
WHERE TABLE_SCHEMA = '%s'

Which could cause a problem if the schema contains the single quote character ' (It's indeed a vaild character in database/table name), we should use the placeholder ? and pass the schema as an argument to db.QueryContext(...) to avoid this kind of problem. Also to avoid the SQL injection problem as user can pass anything to --collect.info_schema.tables.databases in reality (accidentally or on purpose).

[cristiangreco]

Thanks! I've just encountered something similar myself.