Switch to use rhel62-large distro for concurrency* tasks on Enterprise RHEL 6.2 (InMemory) builder

We're seeing OOM failures with the InMemory storage engine occur consistently after the changes from 2f67f3c as part of SERVER-33161. Changing to the rhel62-large distro for the concurrency* tasks on the Enterprise RHEL 6.2 (InMemory) builder is a stopgap for getting the build back to being green until whether the increased memory consumption can be declared as "expected".