Fixes output from generate_symmetric_key_files

When using the Heroku config (ENV vars) for encryption,
generate_symmetric_key_files would output the key as the iv when no iv
was included in the symmetric-encryption.yml file. This change simply
outputs the correct value.
Handle a missing encrypted_key or iv for keygen

Previously if you tried to use encrypted_key/encrypted_iv and had not
loaded them into the ENV (or wherever they will come from) yet, you
would not be able to generate new keys in that environment using ```rails generate
symmetric_encryption:new_keys``` because the Cipher would error out.

This change handles missing encrypted_key or encrypted_iv the same way
we handle missing files when using key_filename or iv_filename in a
given environment.

This way you can actually run the generator in the target environment
without it blowing up.
Merge branch 'handle-missing-encrypted-key-or-iv-in-config'