Add support for encrypting the `sensu` data bag, for SSL and passwords

Passwords are stored in the `secrets` item of the data bag, with the same
structure as the sensu node attributes, ie - if the node attribute for a
desired encrypted value is `node.sensu.dashboard.password`, the data bag
location would be `sensu.secrets.dashboard.password`.

SSL certs stay at `sensu.ssl`, but should be uploaded via knife with the
`--secret-file`/`--secret` argument.