Consider changing default memory reporting to total usage

Currently, by default, `nomad alloc status` reports RSS memory usage; which is different from Docker (that uses memory.usage) and potentially other tools.

Research and consider the appropriate default metrics to show, and update.

Originally reported internally by @notnoop

Start at the `nomad alloc status` command and inspect how its default memory metric is selected and displayed. Compare RSS with Docker's `memory.usage`, determine the appropriate default, and update the command and its coverage so the chosen metric is reported consistently.

Written by the indexing model from the issue text.