Add allowMajorityReadConcern:false evergreen variant

The allowMajorityReadConcern flag exists on 3.6, but is not well-tested. We should add an Evergreen variant identical to "! Enterprise RHEL 6.2", but that runs all mongod instances with allowMajorityReadConcern:false. This will not be a required variant. We will blacklist all tests that require majority readConcern.