stage/systemd-unit:stage to create systemd unit file

Add systemd unit files in osbuild stage

This stage creates systemd unit file in `/usr/lib/systemd/system/`.
The stage accepts filename which must end with `.service`.Section
`Unit` , `Service` , `Install` accepts various parameters as per
the systemd documentaion.`systemd-analyze verify` is be performed
after the .service file is created to check for potential errors.

Signed-off-by: Sayan Paul <paul.sayan@gmail.com>

test/data/stages/systemd.unit.create/a.mpp.yaml

@@ -0,0 +1,31 @@
+version: '2'
+pipelines:
+  - mpp-import-pipelines:
+      path: ../manifests/fedora-vars.ipp.yaml
+  - mpp-import-pipeline:
+      path: ../manifests/fedora-build-v2.ipp.yaml
+      id: build
+    runner:
+      mpp-format-string: org.osbuild.fedora{release}
+  - name: tree
+    build: name:build
+    stages:
+      - type: org.osbuild.rpm
+        inputs:
+          packages:
+            type: org.osbuild.files
+            origin: org.osbuild.source
+            mpp-depsolve:
+              architecture: $arch
+              module-platform-id: $module_platform_id
+              repos:
+                mpp-eval: repos
+              packages:
+                - nftables
+                - openssh-server
+                - systemd
+        options:
+          gpgkeys:
+            mpp-eval: gpgkeys
+          exclude:
+            docs: true

test/data/stages/systemd.unit.create/b.mpp.yaml

@@ -0,0 +1,52 @@
+version: '2'
+pipelines:
+  - mpp-import-pipelines:
+      path: ../manifests/fedora-vars.ipp.yaml
+  - mpp-import-pipeline:
+      path: ../manifests/fedora-build-v2.ipp.yaml
+      id: build
+    runner:
+      mpp-format-string: org.osbuild.fedora{release}
+  - name: tree
+    build: name:build
+    stages:
+      - type: org.osbuild.rpm
+        inputs:
+          packages:
+            type: org.osbuild.files
+            origin: org.osbuild.source
+            mpp-depsolve:
+              architecture: $arch
+              module-platform-id: $module_platform_id
+              repos:
+                mpp-eval: repos
+              packages:
+                - nftables
+                - openssh-server
+                - systemd
+        options:
+          gpgkeys:
+            mpp-eval: gpgkeys
+          exclude:
+            docs: true
+
+      - type: org.osbuild.systemd.unit.create
+        options:
+          filename: create-directory.service
+          config:
+            Unit:
+              Description: Create directory
+              DefaultDependencies: false
+              ConditionPathExists:
+                - "|!/etc/myfile"
+            Service:
+              Type: "oneshot"
+              RemainAfterExit: true
+              ExecStart:
+                - mkdir -p /etc/mydir
+                - touch /etc/myfile
+            Install:
+              WantedBy:
+                - local-fs.target
+              RequiredBy:
+                - multi-user.target

test/data/stages/systemd.unit.create/diff.json

@@ -0,0 +1,7 @@
+{
+  "added_files": [
+    "/usr/lib/systemd/system/create-directory.service"
+  ],
+  "deleted_files": [],
+  "differences": {}
+}