distro/rhel85: tar-installer -> image-installer

Renamed tar-installer to image-installer.
This is a more appropriate name:
- It disassociates the image type from the "tar" image type. The two
  should not be perceived to be connected.
- It's more descriptive. The format of the payload (tar) isn't relevant
  to the purpose of the image type.

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

@@ -240,11 +240,11 @@
       }
     }
   },
-  "tar-installer": {
+  "image-installer": {
     "compose-request": {
       "distro": "",
       "arch": "",
-      "image-type": "tar-installer",
+      "image-type": "image-installer",
       "repositories": [],
       "filename": "installer.iso",
       "blueprint": {}