distro: allow user and group customizations for edge-installer

Followup from, f34380d5b5 and
3a1765a5a8, copied to the rest of the RHEL
distro definitions.

For now, these customizations have no effect on the manifest.

The new `with-users` variants of the edge-installer test cases include
the user customizations in the blueprint, but the manifests are
(currently) the same as the corresponding base cases.

tools/test-case-generators/format-request-map.json

@@ -165,6 +165,52 @@
       "x86_64"
     ]
   },
+  "rhel-edge-installer-with-users": {
+    "compose-request": {
+      "distro": "",
+      "arch": "",
+      "image-type": "rhel-edge-installer",
+      "repositories": [],
+      "filename": "installer.iso",
+      "ostree": {
+        "ref": "test/edge",
+        "url": "http://edge.example.com/repo",
+        "parent": "test/edge"
+      },
+      "blueprint": {
+        "name": "edge-installer-users",
+        "customizations": {
+          "user": [
+            {
+              "name": "user2",
+              "description": "description 2",
+              "password": "$6$BhyxFBgrEFh0VrPJ$MllG8auiU26x2pmzL4.1maHzPHrA.4gTdCvlATFp8HJU9UPee4zCS9BVl2HOzKaUYD/zEm8r/OF05F2icWB0K/",
+              "key": "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC61wMCjOSHwbVb4VfVyl5sn497qW4PsdQ7Ty7aD6wDNZ/QjjULkDV/yW5WjDlDQ7UqFH0Sr7vywjqDizUAqK7zM5FsUKsUXWHWwg/ehKg8j9xKcMv11AkFoUoujtfAujnKODkk58XSA9whPr7qcw3vPrmog680pnMSzf9LC7J6kXfs6lkoKfBh9VnlxusCrw2yg0qI1fHAZBLPx7mW6+me71QZsS6sVz8v8KXyrXsKTdnF50FjzHcK9HXDBtSJS5wA3fkcRYymJe0o6WMWNdgSRVpoSiWaHHmFgdMUJaYoCfhXzyl7LtNb3Q+Sveg+tJK7JaRXBLMUllOlJ6ll5Hod root@localhost",
+              "home": "/home/home2",
+              "shell": "/bin/sh",
+              "groups": [
+                "group1"
+              ],
+              "uid": 1020,
+              "gid": 1050
+            }
+          ],
+          "group": [
+            {
+              "name": "group1",
+              "gid": 1030
+            },
+            {
+              "name": "group2",
+              "gid": 1050
+            }
+          ]
+        }
+      }
+    },
+    "no-image-info": true,
+    "overrides": {}
+  },
   "fedora-iot-commit": {
     "compose-request": {
       "distro": "",