CBQE-1712: support gevented python sdk

changes the underlying sdk implementation to the source in consumer.py,
which uses the python gevented sdk.

Also this change isolates the loader from the worker so that cluster can be loaded
as standalone without celery,rabbitmq,memcached with the --standalone option:

python cbsystest.py run workload --create 100 --ops 80000 --cluster default --standalone

Another feature is that only 1 worker needs to be started in the entire client-ecosystem.
any helper vms will simply need to run ./consumer.py --cluster <name>

Change-Id: I6d0241d13c5e377d08e906ba0b5e40c08402548d
Reviewed-on: http://review.couchbase.org/30429
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>