Add an explanation for ec2_id metadata in images
Add an explanation for ec2_id metadata in images

Change-Id: I76ed8f5cc4c3c4cd15815bfa7d21816e0e21efa8

openstack/ec2-api,stackforge/ec2-api,hayderimran7/ec2-api,hayderimran7/ec2-api,openstack/ec2-api,stackforge/ec2-api