EC2: Add support for images to tags-api

Added EC2 'tags' support for images. The image metadata are used as
tags. We are excluding 'kernel_id', 'ramdisk_id', architecture,
'auto_disk_config' from the metadata while describe tags gets called.

Change-Id: I53240b87f2f371298ca96587b9ccbae0065a5014
Implements: blueprint ec2-tags-api-for-images