Return client tokens in EC2 DescribeInstances

EC2 instances can be instantiated with a client token, a per-instance
unique token that enforces idempotency for RunInstance calls.
If a client token exists, DescribeInstances will return it wrapped
in <clientToken></clientToken>.

Change-Id: I1deb2c0666d0274ed3c181ecb0e0dc65112605ec
Closes-Bug: #1260853
Merge "Return client tokens in EC2 DescribeInstances"