Suggestion: add default values to flux config get

[kkier]

Current behavior of flux config get appears to show only "actively" configured settings (ones that have been set via config files) and ignores defaulted settings. For predicting expected behavior it would be useful to see all the settings, especially when comparing across systems. As a bonus, this also ends up being a handy catalog of all the knobs we have to tweak the system in the config files.