CHEF-4011 - default location of "encrypted_data_bag_secret" should be set in Chef::Config

* moved default encrypted data bag secret file path to `Chef::Config[:encrypted_data_bag_secret]`
* added `--secret` and `--secret-file` options to `knife bootstrap`. This approach is more explicit than the previous approach of reading the path to the secret key from the `encrypted_data_bag_secret` value in the knife.rb file. For backward compatibility we will still attempt to load a key from Chef::Config[:encrypted_data_bag_secret].
* Updated all bootstrap templates to properly read secret from bootstrap context object.
* Added deprecation warning to `knife bootsrap` that is displayed if a user has an 'encrypted_data_bag_secret' entry in their knife.rb file.
* Full test coverage for the above.
* Modernized associated RSpec examples.

Individual commits contain in depth detail about each change.