Generate self-signed certificates using 'sha256' digest (#3777)

* Update certgen.py (default to sha256, md5 is unsecure)

Update to https://github.com/pyca/pyopenssl/blob/d52975cef3a36e18552aeb23de7c06aa73d76454/examples/certgen.py

* Update self-signed certificate generator

sickbeard.helpers.create_https_certificates