Fix sharding with hidden secondaries.

MongoDB's addShard command fails if the connection string includes hidden members. See <https://jira.mongodb.org/browse/SERVER-9882>.

Omit hidden members from connection strings to work around the problem.

I'll issue a pull request against [distribution-devops](https://github.com/Kabam/distribution-devops) to bump the cookbook version in run lists there shortly. I'll also be sending a pull request [upstream](https://github.com/edelight/chef-mongodb) to fix this.

https://www.pivotaltracker.com/story/show/60380166