Remove the generated flag for access token id

The generated flag is used to indicate if the id is automatically generated
by the backend store. If it's set, the data type will be updated when the
model is attached to a datasource.

The AccessToken model defines a string id, which is set in the beforeCreate
hook. So it's client provided id.
Merge pull request #164 from strongloop/feature/fix-accesstoken-id

Remove the generated flag for access token id