Add causal consistent passthrough test.
run jsCore to mongos with setCausalConsistency(true)
1. buildscripts/resomkelib/testing/fixtures/shardedcluster.py: add an c-tor option rs_num_nodes that if present should be passed to _new_shard and start a ReplicaSetFixture instead of standalone shard.
2. add jstests/libs/override_methods/set_causal_consistent.js that may override Mongo or DB methods to set read_preference to secondary and calls
3. add buildscripts/resmokeconfig/suites/causal_consistent_jscore_passthrough.yml file that is a based on sharded_collections_jscore_passthrough.yml
that passes the rs_num_nodes=3 to the ShardedClusterFixture.