Support per-node Nixpkgs overrides and local deployment

Also renamed the `network` key to `meta`.

Cargo.lock

@@ -85,7 +85,9 @@ dependencies = [
  "console",
  "futures",
  "glob",
+ "hostname",
  "indicatif",
+ "libc",
  "log",
  "quit",
  "serde",
@@ -244,6 +246,17 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "hostname"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c731c3e10504cc8ed35cfe2f1db4c9274c3d35fa486e3b31df46f068ef3e867"
+dependencies = [
+ "libc",
+ "match_cfg",
+ "winapi",
+]
+
 [[package]]
 name = "indicatif"
 version = "0.15.0"
@@ -301,6 +314,12 @@ dependencies = [
  "cfg-if 0.1.10",
 ]
 
+[[package]]
+name = "match_cfg"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ffbee8634e0d45d258acb448e7eaab3fce7a0a467395d4d9f228e3c1f01fb2e4"
+
 [[package]]
 name = "memchr"
 version = "2.3.4"