healthcheck: add possibility to check labels to select loopback IP

This allows one to use several instances of the healthchecker, each
instance handling a subset of the loopback IP. This is compatible only
with Linux currently. The documentation at the top has been updated for
that.
Merge pull request #185 from vincentbernat/feature/healthcheck-use-labels

healthcheck: add possibility to check labels to select loopback IP
Merge branch 'master' of github.com:Exa-Networks/exabgp