Fix exception error when creating audit without audit template
Fix exception error when creating audit without audit template

When there is no audit template, data['audit_template'] raises
KeyError. Use data.get('audit_template') instead of
data['audit_template'].

Change-Id: Ic6bcaa8834cbfb6191ce1674a53895140605bf07
Closes-Bug: #1681818

openstack/watcher-dashboard,openstack/watcher-dashboard,openstack/watcher-dashboard,openstack/watcher-dashboard