allow to define mongodb driver ssl system properties in yml configuration file

From https://github.com/SoftInstigate/restheart/issues/88

in order to connect to database server that is using SSL certificate file (PEM) provided by the client, I need to set the System Properties before-hand

Please add an easy implementation of this by adding the following settings in your properties file (maybe at YAML restheart.yml); I believe this will ease lot of people that don't have background on SSL with JAVA.

• javax.net.ssl.keyStore
• javax.net.ssl.keyStorePassword
• javax.net.ssl.keyType
• javax.net.ssl.trustStore
• javax.net.ssl.trustStorePassword
• javax.net.ssl.trustStoreType