Context
The completion of SPM-4330 introduced the multi-router to dispatch command randomly against a pool of mongos  to guarantee good coverage on stale mongos.

Problem
If left black, the number of mongos currently defaults to one. Which means a new engineer unaware of the multi routing property or requirement might leave it empty, causing to deploy 1 mongos and losing possible important coverage. 

Proposal
We should default ShardedClusterFixture to have 3 mongos. 
Ideally, only suites that have a strong reason to keep 1 mongos only should explicitly state it in the configuration file, like in this example  

We can later remove all the num_mongos: x from any suite that uses  ShardedClusterFixture  where x >= 2