Data bag support

I found it very useful to be able to retrieve users public keys from a central data dag, even when setting up users using the LWRP.

This pull requests adds support for not only specifying ssh keys in the `ssh_keys` attribute, but also transparently use data bag items, containing these keys. 

They can be used alongside each other, and the implementation shouldn't break any existing setups.

The following example will assign all three provided keys to the user

```ruby
user_account 'chris' do
  home '/home/chris'
  ssh_keys [ 'ssh-rsa AAAA....' , 'chris' ]
  data_bag 'ssh_public_keys'
end
```

```json
{
  "id": "chris",
  "keys": [
    "ssh-ed25519 AAAA...",
    "ssh-rsa AAAA..."
  ]
}
```