refactor(nixos): make ZFS configuration optional

- move ZFS and snapshot manager settings out of the global base
- group snapshot files under common/optional/zfs
- enable the ZFS module on existing ZFS hosts
- make the global tmpfs setting overridable

common/optional/zfs/snapshot_config.toml

@@ -0,0 +1,29 @@
+["default"]
+15_min = 8
+hourly = 24
+daily = 0
+monthly = 0
+
+["root_pool/home"]
+15_min = 8
+hourly = 12
+daily = 1
+monthly = 0
+
+["root_pool/root"]
+15_min = 8
+hourly = 24
+daily = 0
+monthly = 0
+
+["root_pool/nix"]
+15_min = 4
+hourly = 0
+daily = 0
+monthly = 0
+
+["root_pool/var"]
+15_min = 8
+hourly = 24
+daily = 30
+monthly = 6