Rather than an top-level `allow_userdata` key, instead use a `user_data`
dict. This is to better align with the top-level `vendor_data` keys.

Cloud-init user data often contains user secrets including passwords
and private keys. This data has always been submitted in plain text.
To protect this data's confidentiality and guarantee its authenticity,
this commit add the ability to have this data encrypted and signed.

A new user data format is added allowing for an ASCII armored PGP
MESSAGE. If detected, cloud-init will import into a temporary keyring
any keys provided in /etc/cloud/keys and use these keys to decrypt
and/or verify the provided data.

After decryption, the resulting message will be treated as user data
as before.