Support Client Token for EC2 RunInstances

The EC2 API allows users to provide a unique ClientToken in a RunInstances
call. Future invocations of RunInstances with the same ClientToken return
the same response but do not create new instances.

Fixes bug 1188327.

Change-Id: I88d40db3c04355a752474217cdb184cd386d060c
Merge "Support Client Token for EC2 RunInstances"