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.

TODO:

* Update any existing `_node_*.rb` recipes to use  new resources. We may want to fold this logic into the underlaying slave sub-classes.
* Delete defunct `jenkins_node` LWRP and associated Chef assets (files, templates etc).

/cc @sethvargo @yzl