Remove unused nova client functions.

Remove unused nova client functions.

Following funtions are not required by shade client also
not been used in any of the scenario files:
*create_aggregate
*create_aggregate_with_host
*create_instance
*add_host_to_aggregate
*create_aggregate_with_host
*get_availability_zones
*get_availability_zone_names
*remove_hosts_from_aggregate
*remove_host_from_aggregate
*delete_aggregate
*get_image_by_name
*check_status

JIRA: YARDSTICK-1088

Change-Id: I7ba0fbce6aa3e636aa55581a993c1ec1b48fbead
Signed-off-by: Shobhi Jain <shobhi.jain@intel.com>