Fix issues handling trust tokens via ec2tokens API

Trust scoped tokens are handled incorectly when making requests
via the ec2tokens API, meaning that the restrictions enforced
by trust-scoped tokens are not respected when obtaining a token
via ec2token signature validation.

Storing the trust_id in the blob associated with the ec2 keypair,
and passing that id in the metadata when requesting a v2 token
solves the issue.

Change-Id: I52566384d7813ef0e2f20fb94a5076386457ff02
Closes-Bug: #1242597
Merge "Fix issues handling trust tokens via ec2tokens API"