deposit: doi syntax validator improvements

NOTE the name of the validator was changed to DOISyntax validator
because of convertion to a class. 

@lnielsen-cern I think you use it, so you need to adapt the code.
in order to keep the single line message as in https://github.com/inveniosoftware/invenio/pull/2433/files#diff-ba026b84a5c248720661de03c70296deL146, you can use following syntax:
```python
    self.message = message if message else (
           "The provided DOI is invalid - it should look similar to "
           "'10.1234/foo.bar'.")
```