Add test for bad data bags behavior

This test adds an unrelated encrypted data bag, with unencrypted lumberjack keypairs, reproducing the issue described in https://github.com/rackspace-cookbooks/elkstack/issues/77.
Force missing/bad data bag to throw an exception

By accessing data bags, they will raise the intended exception and handling logic we wanted in the first place.

Fixes https://github.com/rackspace-cookbooks/elkstack/issues/77.
Stub the to_hash databag call, for other tests
Merge pull request #78 from martinb3/master

Fix an issue with unencrypted data bag access