Add Read Preference and Tags to Mongo config

[danny-cassidy]

Rather than rely on a MasterHost config value, which can change during elections, it would be more flexible to use Mongo's built in support for read preference and tags to constrain which nodes in the Hosts field can be used.

If these configurations are set, the MongoDB object can supply these to the mgo Session object before returning it to the client via:

https://godoc.org/gopkg.in/mgo.v2#Session.SetMode (Read Preference)
https://godoc.org/gopkg.in/mgo.v2#Session.SelectServers (Tags)

[jprobinson]

Sounds like a good idea. I usually end up calling SetMode right after calling Gizmo's config.MongoDB.Must() anyways.

@danny-cassidy do you want to take a whack at adding these for your first Gizmo PR? If not, I can find some time to do this in the next day or two.

[danny-cassidy]

Yeah, I'd love to submit a PR for this. 👍

[jprobinson]

👍 Looking forward to it!

[jprobinson]

This was resolved in PR #54