Report both total actual system memory and cgroup memory limit in hostInfo

WIth SERVER-16571 hostInfo.memSizeMB will now report the cgroup memory limit instead of the actual system memory. However system memory statistics in ftdc are reported for the system as a whole, not just for the container, so interpreting them requires knowing the total actual system memory. There may be other diagnostic purposes for knowing the actual system memory as well, so it's difficult to know up front the impact of no longer having this information. Also, seeing different values for system memory vs memSizeMB will alert us to the fact that a cgroup is in use, which also has diagnostic value.

Suggest adding a field, maybe hostInfo.systemMemSizeMB, to report this information.