Add support for Mongo Cluster

[bufke]

Currently it's not clear how to configure kobocat for a mongo cluster and the connection may be unstable. It's unclear if we need to make code changes, upgrade pymongo, or only change the settings configuration.

In a test with the mongo helm chart, using the connection string with the "headless" service resulted in NotPrimaryError errors. Errors appeared at random.

with adding ?replicaSet=rs0 worked for some time without errors. Then consistently started to get ServerSelectionTimeoutError. In this test, kobo worked even after forcing downtime and failover on the mongo cluster. The error took some time to manifest but then happened very consistently.

[jnm]

Related to #837, whose original description was about making code changes to retry failed MongoDB operations

[j-manu]

MongoDB cluster has mongodb+srv:// URIs and it can be supported by adding dnspython as a dependency. Latest version of pymongo has this as a required dependency. I'm currently using Kobo with a mongodb cluster after installing dnspython.