CXX-357 test against multiple versions of mongod

I think you could use expansions to avoid having these all spelled out, since many of them are identical across different builds.  For example, ```mongo_url_enterprise_keyword``` is "enterprise-" for everything except the RHEL55 32-bit build.  It might be nicer to set ```mongo_url_enterprise_keyword: ""``` explicitly on that buildvariant, and then within the "set version x" tasks below, use ```${mongo_url_enterprise_keyword|"enterprise-"}``` so that the default is "enterprise-" and we don't have to define that for every system.  I would say the same for ```mongo_url_subscription_keyword``` and ```mongo_url_extension```