adopt required chefspec version
Test searching for clients
mock out nodes, try to mock out an environment with nodes and find it with a multi_environment_monitoring
Tests for the multi_environment_monitoring

Two nodes are added inside the 'offsite' environment.

These should be found when multi_environment_monitoring is true.
They shouldn't be found when multi_environment_monitoring is not set and the node itself is in a different environment.
make rubocop happy
Merge branch 'add_server_search_tests' of github.com:bcs-de/munin into add_server_search_tests
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.
Test the new multi-environment search for clients

Add a test to find only clients in specific environments.
Had to redo the stubbing to put nodes into an environment.
make rubocop happy
adopt the README
Merge branch 'fix_multi_environment' of github.com:bcs-de/munin into fix_multi_environment
Merge branch 'fix_multi_environment' into feature/graph_strategy