RAM & Disk Usage gauges are not always accurate or clear

This issue consolidates a few (need to find and link them here) issues raised in the past about misleading and confusing reporting of usage metrics in the UI. There are two main areas of concern:

1. how to show proper metrics for services vs. buckets vs. cluster (collections?)
2. how many data points to include in a single gauge (e.g. should bucket RAM use show what's in use by other buckets on the same gauge?)

As for implementation, it may well be that:

1. our current set of API calls are insufficient/inaccurate.
2. the current set of directives for (mn_bar_usage) displaying the metrics is too limited