Check if max_attempts is None for ConstantReconnectionPolicy

[thoslin]

Found some issues with this reconnect policy:

class ConstantReconnectionPolicy(ReconnectionPolicy):
    """
    A :class:`.ReconnectionPolicy` subclass which sleeps for a fixed delay
    inbetween each reconnection attempt.
    """

    def __init__(self, delay, max_attempts=64):
        """
        `delay` should be a floating point number of seconds to wait inbetween
        each attempt.

        `max_attempts` should be a total number of attempts to be made before
        giving up, or :const:`None` to continue reconnection attempts forever.
        The default is 64.
        """
        if delay < 0:
            raise ValueError("delay must not be negative")
        if max_attempts < 0:
            raise ValueError("max_attempts must not be negative")

        self.delay = delay
        self.max_attempts = max_attempts

    def new_schedule(self):
        return repeat(self.delay, self.max_attempts)

If passing max_attempts=None to ConstantReconnectionPolicy would raise a value error, since None is smaller than 0:

In [1]: from cassandra.policies import ConstantReconnectionPolicy

In [2]: policy = ConstantReconnectionPolicy(10, max_attempts=None)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)


<ipython-input-2-763f0e978a2e> in <module>()
----> 1 policy = ConstantReconnectionPolicy(10, max_attempts=None)

/home/tom/Workspace/polaris-storage/.venv-storage/local/lib/python2.7/site-packages/cassandra/policies.py in __init__(self, delay, max_attempts)
    511             raise ValueError("delay must not be negative")
    512         if max_attempts < 0:
--> 513             raise ValueError("max_attempts must not be negative")
    514 
    515         self.delay = delay

ValueError: max_attempts must not be negative

And repeat doesn't take None as an argument:

In [3]: from itertools import repeat

In [4]: repeat(10, None)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-78a54565c583> in <module>()
----> 1 repeat(10, None)

TypeError: an integer is required

[aholmberg]

Thanks! Created to track for testing: https://datastax-oss.atlassian.net/browse/PYTHON-325