Adds bin/certs-openssl, which creates self-signed root cert/key and issuer cert/key using openssl. This will be used in the two clusters set up in the multicluster integration test (followup PR), given CI already has openssl and to avoid having to install step.
Adds a new flag `--certs-path` to the integration tests, pointing to the path where those certs (ca.crt, ca.key, issuer.key and issuer.crt) will be located to be fed into linkerd install's `--identity-*` flags.
Alejandro Pedraza