Keep fewer logs around

[hacklschorsch]

Refs #17

[btlogy]

LGTM

I'll also back port those change here:

infrastructure/nix/common/default.nix

Lines 8 to 10 in d878ca0

	services.journald.extraConfig = ''
	SystemMaxUse=500M
	'';

[hacklschorsch]

@btlogy I am won't be including our global defaults in the test grid definition - I want this to stand on its own (as an example for someone else wanting to stand up a grid like this). OK?

[btlogy]

@btlogy I am won't be including our global defaults in the test grid definition - I want this to stand on its own (as an example for someone else wanting to stand up a grid like this). OK?

I'm not sure to see much value into keeping it on its own. But this will lead to repetitions.
Also, it a contributor likes the testgrid the way we propose to manage all other system, it will be easier for him to PR for other systems too.
I know you like to keep things contained as much as possible. We can always change this later if it hurts.

But than I wonder how much contained: no flake, no CI and no CD for testgrid?
Maybe we should have a repository for each system then?