Mock OCSP server uses openssl command

[garethsb]

As mentioned on https://github.com/AMWA-TV/nmos-testing/blob/6800d535376f5cd6fb2aee51642389efd02da75a/docs/1.1.%20Installation%20-%20Local.md#dependencies, mock OCSP server uses openssl command.

nmos-testing/nmostesting/OCSP.py

Lines 39 to 44 in eff038d

	subprocess.run(["openssl", "ocsp", "-index", "test_data/BCP00301/ca/intermediate/index.txt",
	"-rsigner", "test_data/BCP00301/ca/intermediate/certs/intermediate.cert.pem",
	"-rkey", "test_data/BCP00301/ca/intermediate/private/intermediate.key.pem",
	"-CA", "test_data/BCP00301/ca/intermediate/certs/ca-chain.cert.pem",
	"-reqin", "test_data/BCP00301/ca/ocspreq.der",
	"-respout", "test_data/BCP00301/ca/ocspresp.der"])

I think it would be better if the same could be achieved using the python OpenSSL library... or we should update check_external_requirements function to check it is installed.

nmos-testing/nmostesting/NMOSTesting.py

Line 982 in bf723e8

def check_external_requirements():