Configure inputs outputs

**Replaces PR #12**

This commit revises some of the work in PR #12. Primarily I want to ensure that the configuration is as dynamic and flexible as possible. I do want to avoid attribute sprawl as much as possible, as that creates a large cognitive load for people consuming the cookbook.

I also updated the test coverage. Details:

* Create a new kitchen suite specifically for testing the inputs/outputs configuration.
* Corrected a mispelled directive for inputs
* Revise the README with examples for the attributes
* Rename attributes introduced in PR #12 to clarify their intention.
  + Rename `node['splunk']['tcpout_server_config_map'] to
  `node['splunk']['outputs_conf']`.
  + Rename `node['splunk']['input']` to
  `node['splunk']['inputs_conf']`.
* Move static config from outputs.conf.erb to default attributes in
  `outputs_conf`.
* Fix deprecated `#any_instance` method in spec
* Create a separate context in ChefSpec for the inputs/outputs and set
  the node attribute before the tests.
* Convert to latest ServerSpec (set backend, don't need
  Serverspec::Helper modules)