debian: Delete log files correctly when packages are purged.

Debian Policy 10.8 says "Log files should be removed when the package is
purged (but not when it is only removed)," but the Open vSwitch packages
didn't properly implement this:

   - The openvswitch-brcompat and openvswitch-pki packages didn't delete
     their log files at purge time.

   - The openvswitch-switch package deleted all of the log files at purge
     time, even those owned by openvswitch-brcompat or openvswitch-pki.

This commit fixes both problems.

CC: 656448@bugs.debian.org
Reported-by: Andreas Beckmann <debian@abeckmann.de>
Signed-off-by: Ben Pfaff <blp@nicira.com>