CLI list/show subcommands should be consistent between types

[jeremyh]

The outputs of the list/show subcommands are quite different in format and arguments, and some of their outputs are not currently very useful. We should make them more consistent for a better user experience.

datacube dataset [list|show]
datacube product [list|show]
datacube metadata_type [list|show]
datacube user [list|show]

[jeremyh]

To clarify, I think there's two big reasons to have the cli tools in addition to the python api, which we should emphasize in the design:

1. Convenience: not needing to fire up a python environment when working in a shell. Most housekeeping/setup tasks are done from shell rather than python in our current docs & usage.
2. Ability to easily use these tools together with the rest of the cli/bash/excel/etc software ecosystem. Many of the current output formats are currently not very convenient for piping to other tools. Ability to parse/extract values from the output is a plus. The dataset commands have a --format specifier for getting the output in parse-able formats like csv, which traditionally Ops used quite heavily.