Cosmetics changes

Improve test case description.
Use the new NFS image-deployment value.

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>

test-cases/nfsroot-simple-boot.yaml

@@ -4,13 +4,12 @@ metadata:
   image-types:
     nfsroot: [ armhf, arm64, amd64 ]
   image-deployment:
-    - APT
-    - OSTree
+    - NFS
   type: functional
   exec-type: automated
   priority: high
   maintainer: "Apertis Project"
-  description: "Check that the image booted."
+  description: "This test checks that an image that was flashed using the nfsroot  properly booted."
 
   expected:
     - "The output should show pass."