nixos: openstack: have its own metadata fetcher expression

These two APIs have diverged over time and are no longer compatible.

nixos/modules/virtualisation/openstack-config.nix

@@ -3,7 +3,7 @@
 with lib;
 
 let
-  metadataFetcher = import ./ec2-metadata-fetcher.nix {
+  metadataFetcher = import ./openstack-metadata-fetcher.nix {
     targetRoot = "/";
     wgetExtraOptions = "--retry-connrefused";
   };

nixos/modules/virtualisation/openstack-metadata-fetcher.nix

@@ -0,0 +1,23 @@
+{ targetRoot, wgetExtraOptions }:
+''
+  metaDir=${targetRoot}etc/ec2-metadata
+  mkdir -m 0755 -p "$metaDir"
+
+  echo "getting EC2 instance metadata..."
+
+  if ! [ -e "$metaDir/ami-manifest-path" ]; then
+    wget ${wgetExtraOptions} -O "$metaDir/ami-manifest-path" http://169.254.169.254/1.0/meta-data/ami-manifest-path
+  fi
+
+  if ! [ -e "$metaDir/user-data" ]; then
+    wget ${wgetExtraOptions} -O "$metaDir/user-data" http://169.254.169.254/1.0/user-data && chmod 600 "$metaDir/user-data"
+  fi
+
+  if ! [ -e "$metaDir/hostname" ]; then
+    wget ${wgetExtraOptions} -O "$metaDir/hostname" http://169.254.169.254/1.0/meta-data/hostname
+  fi
+
+  if ! [ -e "$metaDir/public-keys-0-openssh-key" ]; then
+    wget ${wgetExtraOptions} -O "$metaDir/public-keys-0-openssh-key" http://169.254.169.254/1.0/meta-data/public-keys/0/openssh-key
+  fi
+''