KNIFE-494 add options for secret and secret_file to support encrypted data bags

When using encrypted data bags, the current plugin uses the soon to be deprecated encrypted_data_bag_secret and encrypted_data_bag_secret_file options. This patch updates knife openstack server create to use the same functionality as knife bootstrap. Also it allows for command line switches to use this functionality instead of having to embed it in the knife.rb file.

The warning included was copied directly out of knife bootstrap.