d/gestioip.postinst: update needed apache2 conf in postinst

Apache2 in Debian no longer gives the dir /etc/apache2/conf.d.

So, let's deploy our apache2 conf file to /etc/apache2/sites-available
and then call a2ensite to enable the webapp.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>