Move vpn_image_id to pipelib

Apart from checking whether a given image is the cloudpipe image, the
vpn_image_id option is only used within pipelib itself.

Add a is_vpn_image() helper method and move the option into pipelib.
Some rejiggering of how pipelib imports ec2 opts is required to avoid
circular imports.

blueprint: scope-config-opts
Change-Id: Ie984b2bb81681c24d3cee803082960083992a535
Merge "Move vpn_image_id to pipelib"