Added global config option to allow configuration of ssl settings

[alexbechmann]

This PR allows you to use CosmosDB Mongo instance on azure which requires TLS1.2.

Usage: ` MongoRepository.GlobalConfig.ConfigureSslSettings(x => x.EnabledSslProtocols = System.Security.Authentication.SslProtocols.Tls12);

IRepository<NewsArticleDocument> repository = new MongoRepository<NewsArticleDocument>(ConfigurationManager.ConnectionStrings["MongoServerSettings"].ConnectionString, collectionName: "news-articles"); `