bugfix: repeated ping checks cause memory leak
bugfix: repeated ping checks cause memory leak

Every call to the ping endpoint resulted in creation of new
Session and Cluster objects by the cassandra driver. Instead, we
re-use the same connection to perform the health check. See
https://datastax-oss.atlassian.net/browse/PYTHON-135 for reference.

Change-Id: I879debe227ef88438e9cbd1dcb6104a7f3ed79a9

openstack/poppy,stackforge/poppy,stackforge/poppy,openstack/poppy,stackforge/poppy,openstack/poppy