heat api : don't store whole request in ec2Credentials context

The ec2token middleware currently dumps the whole request into the
request context when using AWS Auth, when all we really want are
the credentials (accesskey/signature)

fixes bug 1072917

Change-Id: I8fb0d4d7ada29d7d19c702a7da1251384166c246
Merge "heat api : don't store whole request in ec2Credentials context"