Add helper method to get the virtual hostname of haproxy in the cluster

This is the simple code substituting _ with -, but it will be used in
several places, so we should have a method for this.
Allow specifying an arbitrary public name for the public virtual IP

This is important because end users will see the public FQDN
(cluster-foo.domain, with foo being the name of the pacemaker proposal),
which can be considered ugly but also an implementation detail. This
happens for endpoints, for instance.

The public FQDN also matters for SSL certificates, since the SSL
certificates need to be valid for that hostname.

So if we want to hide this public FQDN, having the cloud operator define
a CNAME in the DNS is not enough, we also need to have the barclamp know
about the public name picked by the cloud operator. Hence this new
setting.

This is obviously HAProxy-specific.
Add sections to barclamp view to split settings more logically
Add a big info box about generated hostnames for virtual IPs

This is important to know when creating SSL certificates.
Merge pull request #39 from vuntz/public-name

Add ability to specify a public name for haproxy public virtual IP