Add ability to supply cert file

[aviaviavi]

Documenting an issue uncovered by @flokli in a private repo that I'll describe here:

curl-runnings uses http-client-tls, which apparently has no nice way to configure your own cert file.

@flokli opened snoyberg/http-client#461, so http-client-tls honors SSL_CERT_FILE.

He recommended Switching Network.HTTP.Client.TLS to Network.HTTP.Client.OpenSSL and adjusting noVerifyTlsSettings should be sufficient