add debconf tool to manage postgresql.conf file modification

During the installation of the package if the postgresql.conf already exists and the 'standard_conforming_strings' is set to 'on' a question is asked to the user using debconf tool to be authorized to modify its value to 'off'.

To test this feature you must:
- create a new ubuntu python-oq-engine package
- set 'standard_conforming_strings'  to 'on' 
- install python-oq-engine package