Less swap on disk constrained AWS hosts

This configures less swap space on AWS hosts (4 GB instead of 8 GB) since I learned that those only have 20 GB of disk space.

My goal of having some time to react if swap begins to fill up shouldn't suffer much from having 4 GB instead of 8 GB. This MR will result in different hosts having a different amount of swap space (some 4, some 8 GB). Chaos, I know! But I could totally handle that. If we rather make all the hosts equal on swap (why - they also aren't on RAM..) I propose we set swap to 4 GB everywhere.

I don't want an by-design unused swap file taking up 40 % of our AWS instances local disk space. That's just a lot of resources for a (discussable..) safety net.

morph/lib/issuer-aws.nix

@@ -9,7 +9,7 @@
   boot.kernel.sysctl = { "vm.swappiness" = 0; };
   swapDevices = [ {
     device = "/var/swapfile";
-    size = 8192; # megabytes
+    size = 4096; # megabytes
     randomEncryption = true;
   } ];