[Python 3]no debug output for ldap and sql

Summary:
Error: no debug output in Python3 for ldap and sql

var conf.debuglevel = 9 not set within "-d 9"

checked in pykolab/auth/ldap/__init__.py line 414 so trace_level is not set

main cause:
retval is in all cases set as False, as it is not set within exec() to True.
this don't throw any Python Error, but could be seen in debug output, as debuglevel is 0 also it is set on cli to 9

exec is statement in 2.7, changed to function in 3.x

It doesn't need a code block execution here, so eval() does the job, too.
eval() will lower the risk of a code injection.

Reviewers: PyKolab Developers, sicherha

Reviewed By: PyKolab Developers, sicherha

Differential Revision: https://git.kolab.org/D3620