Expose pymongo's SSL cert options

Expose 'ssl_keyfile','ssl_certfile','ssl_cert_reqs' and
'ssl_ca_certs' options for maximum security. By default, ssl
is not enabled except that ssl parameter was included in the
mongodb uri directly, and ssl_cert_reqs = CERT_REQUIRED which
means user must provide the 'ssl_ca_certs' if ssl is enabled
by adding the ssl parameter in the mongodb uri.

Change-Id: I67cb5a9b2d76625de2932c854d0a696e9118ca6b
Closes-Bug: #1328720
Merge "Expose pymongo's SSL cert options"