Adopt the multi_environment_monitoring

One could use the multi_environment_monitoring-parameter to be [false, true, array_of_string]. When its false, only the own environment is searched. When its true, all environments are searched. And when its a list of environment-names, the named environments are searched.
make rubocop happy
Update README.md

Add a list of allowed values to the attributes description and a bigger piece of text to the usage section to describe what multi_environment_monitoring can be used for.
Merge pull request #32 from bcs-de/feature/extend_multi_environment_monitoring

Extend multi environment monitoring