[BUGFIX] Fix SQL error in BackendConfigurationManager

The first argument for exec_SELECTcountRows was missing.
[BUGFIX] Fix SQL Injection possibilities

Table name is fetched from GET parameters and
passed directly to a database query.

This change checks the table name against a whitelist
(TCA) and throws an exception if the table name does
not exist.

Additionally another value passed by GET is sanitized
to integer before using in SQL string
Merge pull request #458 from helhum/sqli-fix

Fix BackendConfigurationManager