Document ESP HTTPS cipher list configuration

HPCC-15237 adds the ability to configure the list of ciphers that ESP will use with OpenSSL to negotiate with clients.

I have defaulted to "ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5" for now. But now that its configurable people can play with suggested strings and determine what the default should really be and we can change it.

See:
http://www.acunetix.com/blog/articles/tls-ssl-cipher-hardening/
https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/

And for general OpenSSL cipher information:

https://www.openssl.org/docs/manmaster/ssl/SSL_CTX_set_cipher_list.html

https://www.openssl.org/docs/manmaster/apps/ciphers.html