Per-component default configs

[reidsunderland]

We've discussed this before, but I couldn't find an issue for it. It's possible that an issue already exists.

There are cases where it would be useful to have default configs on a per-component basis. This is something that we had in v2, but was rarely (or never?) used. I think part of the reason it was rarely used was because v2 had different per-component defaults in the code. In sr3, we've mostly done away with that because it was confusing and now have the same defaults across the board.

Because we now (in sr3) have the same defaults for all components in the code, now the per-component defaults might actually be useful.

In v2, you'd put your per-component defaults in ~/.config/sarra/component.conf, like ~/.config/sarra/winnow.conf:

When a component is started up, a series of configuration files are read in the following sequence:

default.conf
admin.conf
.conf (subscribe.conf, audit.conf, etc...)
/.conf

https://github.com/MetPX/sarracenia/blob/v2_dev/doc/sr_subscribe.1.rst#option-order

But we discussed that putting them in the directory might make more sense, like ~/.config/sr3/winnow/default.conf (or maybe default.inc would be better? or easier to implement?). This (putting a default file in a subdirectory) would also be somewhat more in line with eliminating components. #494