PHPLIB-430: Add test environment with multiple mongos nodes

[alcaeus]

https://jira.mongodb.org/browse/PHPLIB-430

This PR changes the mongo-orchestration configs for sharded cluster to include a second mongos node for each. To ensure that we're not running into errors due to running subsequent commands on separate mongos instances, the tests modify the connection URI to only use a single mongos node, unless explicitly requested to use multiple nodes. This is currently only the case in select tests for the retryable-writes and transaction specs.

Note that some tests had to be disabled:

• Transaction tests on these clusters fail due to an error thrown in the driver, see PHPC-1359: Prohibit startTransaction() on sharded clusters mongo-php-driver#984 for more details. Once this has been removed in the driver, we can then run these tests again.
• Retryable write tests are skipped on non-replica set sharded clusters due to PHPC-1415. Once this has been fixed this can be re-enabled.
• As mentioned in Run tests on sharded cluster with Replica Sets #655, functional tests for the Watch operation are disabled on sharded clusters with replica sets because of an issue with getMore not returning information. I'm still investigating that issue and will fix it separately.

[kvwalker]

lgtm!