Fix bug in agent recipe's custom logstash configs

Fix a bug where the agent recipe was not installing custom configuration files with the right instance name (was using 'default' instead of 'agent'). Added a test fixture/wrapper cookbook in order to verify the correct behavior.

Gave existing agent serverspec tests some easier to read description groups, to help read output easier.

Bump version
Fix quoting for rubocop
Merge pull request #69 from martinb3/master

Fix bug in agent recipe's custom logstash configs