Credentials and Slaves oh my!

This PR:

* Tweaks the TK configuration (see individual commits for details).
* Ensure the server/master generated public key is saved to authorized_keys.
* Adds `jenkins_credentials` resources.
* Adds `jenkins_slave` resources.
* Nuke legacy `jenkins_node` LWRP.
* Nuke legacy `_node*.rb` recipes.

/cc @sethvargo @yzl