install-server fix logrotate for squid3

Fix squid3's logrotate to check for the right pidfile. Additionally,
place the logrotate under /etc/logrotate.d/squid3, instead of squid, as
the packaage provides squid3 and having both results in "duplicate log
entry" cronspams.

Change-Id: I274b0fb8ef79dbc68d501a9f39e1de7d52002aff