Fix IntValidator regarding validation of value 0 (#2818)

Previous to this commit `IntValidator` would always consider the value `0` invalid, even if in the allowed range.

The problem was that `parse_int` was returning `0` in the following line:

```python
value = parse_int(value, self.base_unit) or ""
```

However the `or ""` was evaluating to an empty string.

As `parse_int` returns either an `int` if able to parse, or `None` otherwise, the `isinstance(value, int)` is enough to error out when not a valid `int`.

Closes #2817

patroni/validator.py

@@ -793,7 +793,7 @@ class IntValidator(object):
         :param value: value to be checked against the rules defined for this :class:`IntValidator` instance.
         :returns: ``True`` if *value* is valid and within the expected range.
         """
-        value = parse_int(value, self.base_unit) or ""
+        value = parse_int(value, self.base_unit)
         ret = isinstance(value, int)\
             and (self.min is None or value >= self.min)\
             and (self.max is None or value <= self.max)