More idempotency in the EC2 Slave build script.

Change-Id: Ic065a4a4ab2be6bd9323dc63a540a820223e9fae
More idempotency in the cowbuilder bootstrap script.

Change-Id: I09aeff0ab1c0d3346e3a9bc4ef1aa4ed477c14a8
Merge "More idempotency in the cowbuilder bootstrap script."