Ticket/master/pdb 554 Fix zombied instances and volumes for EC2

This contains 2 commits, 1 is to introduce a new backend 'ec2' that its functionally equivalent to blimpy, but removes the need for blimpy completely instead replacing it with aws-sdk. It also adds the tag 'jenkins_build_url' so we can get a trace back as to what job created what image (also added for blimpy and google_compute_engine).

The second commit will now check that an image exists first, copy over its block_device_images settings and enforce that volumes get deleted upon instance termination. This should remove the volume leak we've been seeing in EC2 as well.