Extend multi environment monitoring

This is a backwards compatible extension of the multi-environment munin-server.

Until now one could choose with true or false whether the munin-server should check nodes only in the own environment or nodes in all environments.

With this patch the attribute munin/multi_environment_monitoring can be true or false for the old behavior or a list of environment-names to add only nodes from these named environments.

Replaces #30, COOK-4123 in tickets.opscode.com

The documentation for the attribute in ReadMe.md is missing, will do that next.