Chef 12 bootstrap cleanup

* Cleans up deprecations for Chef 12
* Implements the encryted data bag secret distribution based on [this](https://gist.github.com/sersut/94c8daad5c11369bd2e8) 
* Simplifies the bootstrap CLI options slightly.

@opscode/client-engineers wanted to share this before increasing the spec coverage around these options. 

Remaining work:

* [ ] Moar specs.
* [ ] End to End bootstrap testing
* [ ] CHANGELOG and RELEASE_NOTES updates.
We're combining the behavior of two arguments here, so there's opportunity for surprise. We should at least provide a debug message that contains the full path to the file we're choosing to use due to the argument. Also consider skipping or raising here if template is a directory.