Added metadata.Vars as quick access to Property like attributes

[xschlef]

We needed a plugin that solves the problem of adding additional metadata that cannot be represented as a group:

For example:

<Group name="balanced">
   <Client name="foo.example.org">
        <var name="private_ip">10.0.1.3</var>
   </Client>
   ...
</Group>

This can now be accessed everywhere where a metadata object is present especially when generating configurations for monitoring systems like nagios / icinga.

We thought about mapping this to Properties, but we didn't want to reevaluate the properties everytime a client checks in, as we have an instance with 300+ clients.

Maybe you find this useful.