Use dot notation to access instance object fields in ec2 create_image

The create_image method is using an instance object now so access the
fields using dot notation rather than treating it like a dict.

Also re-uses the in-scope instance_uuid variable rather than
instance.uuid for the name_map dict creation.

Change-Id: Ia80277830dc7230fb113e6f15efc773342ea4572
Merge "Use dot notation to access instance object fields in ec2 create_image"