stage/systemd-unit-create:paremeterize file location

User can now customize the systemd unit load path.
User can select between etc or usr , defaults to 'usr'.
Also user can customize the scope of the service between global
or system, defaults to system.

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

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

@@ -616,6 +616,8 @@
           "type": "org.osbuild.systemd.unit.create",
           "options": {
             "filename": "create-directory.service",
+            "unit-type": "global",
+            "unit-path": "etc",
             "config": {
               "Unit": {
                 "Description": "Create directory",

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

@@ -1,6 +1,6 @@
 {
   "added_files": [
-    "/usr/lib/systemd/system/create-directory.service"
+    "/etc/systemd/user/create-directory.service"
   ],
   "deleted_files": [],
   "differences": {}