Bugfix: Error validating value 'cirros-0.3.5'

Bugfix: Error validating value 'cirros-0.3.5'

JIRA: YARDSTICK-607

some scenarios come with a Cirros-0.3.5 image, Yardstick's load_images.sh will
load a second cirros-0.3.5 image, causing the following error:
2017-03-27 07:42:58,317 yardstick.orchestrator.heat heat.py:449 INFO Creating stack 'demo-2ada98f3'
error: failed to deploy stack: 'ERROR: Property error: : resources.ares.demo-2ada98f3.properties.image: : Error validating value 'cirros-0.3.5': No images unique match found for
{'name': u'cirros-0.3.5'}

This patch fixs this error by skipping load cirros image if there already exist
one in the glance.

Change-Id: I4d43d82d331f6575efd4560be9b6be175e03cd1b
Signed-off-by: JingLu5 <lvjing5@huawei.com>