Allow passing a filesystem path to bind

[jinty]

So you can setup haproxy to listen on a socket. For example:

        haproxy::listen { "listener":
                bind      => {"/path/to/socket" => []}
        }

[underscorgan]

@jinty thanks for the contribution, but this needs documentation and test updates for the new allowed behaviors. Once you make those updates, please squash everything down to a single commit.

[antaflos]

Just like the changes in #152 and #143 this change is also moot because the complex and limiting validation code for the $bind parameter has been removed in #154. Specifying a filesystem path in bind is now totally possible and even tested (both in spec tests and in our production systems).

[underscorgan]

Thanks @jinty , but closing due to @antaflos's comment.