Enable pcr.json as input to oyster-cvm verify-enclave

[prateekreddy]

oyster-cvm currently creates a pcr.json when enclave image is built. It would probably make sense for the user to publish pcr.json directly. In that case anyone who wants to verify the enclave using oyster-cvm verify-enclave should be able to download pcr.json and pass the path to it as an argument to verify.