Extend multi environment servers

Giving the server only the choice of searching clients of the own environment or searchign for _all_ environments, is a bit constricting. What if someone wants to search for clients in multiple given (but not all) environments?

This is a backward-compatible change that allows the parameter ```node['munin']['multi_environment_monitoring']``` to be `true`, `false` or a list of environment names.
If the value is a list of names, these environments are searched for clients.

For example a munin-server in the 'HQ' environment can monitor clients in 'HQ' and 'production', while not searching for clients in environments 'development' od 'staging' when ```node['munin']['multi_environment_monitoring'] = ['HQ', 'production']```.

Depends on / includes #29 (thats how I can prove the backward-compatibility:)