healthcheck: don't advertise community either when withdrawing a route

Only the prefix is advertised in a withdraw message. While exabgp
doesn't have a problem with getting more stuff, it seems cleaner to not
advertise anything else while this is just ignored.

Closes: #184
steal preexec_fn trick from exabgp processes.py, fixes #183
Merge pull request #188 from PowerDNS/healthcheckpgrp

steal preexec_fn trick from exabgp processes.py, fixes #183
Merge pull request #187 from vincentbernat/fix/healthcheck-no-community-on-withdraw

healthcheck: don't advertise community either when withdrawing a route