healthcheck: don't advertise med when withdrawing a route

There is no MED in a withdraw message. While exabgp doesn't have a
problem with that, it seems cleaner to not advertise one while this is
just ignored.

Closes: #184
Merge pull request #186 from vincentbernat/fix/healthcheck-no-med-on-withdraw

healthcheck: don't advertise med when withdrawing a route
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
do not force a next-hop to withdraw
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
allow recording on ibgp test tool
fix python2.6 compatibility undoing a suggestion from pycharm
fix MD5 for IPv6
to allow a bug report