Use PacemakerServiceObject.cluster_vhostname_from_element

We shouldn't compute the hostname of haproxy ourselves, so instead use
the new helpe for this. This will also fix a bug where we need to
substitute _ with -.
Use helper from database cookbook to get database IP address

This way we're ready for when HA will be used for database.
Fix regression with memcache possibly listening on wrong address

With the HA support in commit a541099b, we lost the code ensuring that
memcache only listens where it should (when HA is used; it kept working
fine for non-HA).
Merge pull request #141 from vuntz/cluster-hostname

Fix hostname of cluster to use - instead of _
Merge pull request #142 from vuntz/db-ha

Use helper from database cookbook to get database IP address
Merge pull request #143 from vuntz/memcache-listen

Fix regression with memcache possibly listening on wrong address