images fail to register with --use_project_ca=True

[Vish Ishaya]

To reproduce:

1. set --use_project_ca in flagfile
2. create a user and project
3. get credentials and source them
4. euca-bundle-vol
5. euca-upload-bundle
6. euca-register-image

Result:

Image gets stuck in decrypting state

Expected:

Image should become available after decryption

This is due to the decryption code always using the cloud pk to decrypt instead of the per-project pk.