Add --ignore-connection-issues option

Sometimes we prefer to have unknown status when we have a connection
issue during a test.
Read by default on secondary for collection_check
Add check of indexes existence.

Indexes existence is only done by the name of the indexes. They are
provided using -W and -C as a comma-separated-list of indexes names.
Use thresholds for chunk balance

It cannot be expected from the cluster to be more
balanced than what the policy from MongoDB is.
Add a timeout option

If the timeout is reached, returns an unknown state.