Add SSL context configuration

MongoDB offers a possibility to use SSL client certificates and to use
the root CA to verify hosts. This change makes these settings accessible
from the config by passing a Hash instead of a boolean to the SSL
option.

This patch is heavily inspired by dc21475820ff148fb42963752db0bfa6a23f5e1e
Merge remote-tracking branch 'origin/ssl_options' into ssl_and_auth_fix