[Reviewer: Andy] For a HTTPS Clearwater repo, retrieve the keys from a databag

Andy,

With this change, if the Clearwater repo server is configured as a HTTPS URL, Chef will retrieve keys from an encrypted databag and set APT up to do client-side authentication.

The prerequisites are:
* You need an encrypted databag called repo_keys/generic, with attributes called "repository-ca.crt", "repository-server.crt" and "repository-server.key", each of which is the relevant PEM-encoded certificate or key. (We could enhance this in future, if we need per-environment keys rather than per-Chef-server keys.)
* You need an `encrypted_data_bag_secret` setting in ~/.chef/knife.rb, pointing at the same file used to encrypt the databag.

I'll send an email to the team with our specific secrets once this is merged.