The Eventlet reactor should not do the monkey patch

The eventlet reactor is currently calling `eventlet.monkey_patch()` during its initialization although the documentation mentions:

We should remove that and rely on the user to take care of monkey patching, like the gevent reactor. With this ticket, we will be able to remove this custom check:

https://github.com/datastax/python-driver/blob/master/cassandra/cluster.py#L1095

in favor of https://github.com/datastax/python-driver/blob/master/cassandra/cluster.py#L80