Clarify diskimage names in docs

The docs set up "trusty" and "precise" etc for labels and provider-images,
however in the diskimage section it calls them ubuntu-precise and so on.
This is confusing where the names operate as keys.

Change-Id: I35668427ce77f997c053a3d145dbd7949008aaad

doc/source/configuration.rst

@@ -231,7 +231,7 @@ uploads will be deleted as well as the files on disk.
 Example configuration::
 
   diskimages:
-    - name: ubuntu-precise
+    - name: precise
       pause: False
       rebuild-age: 86400
       elements:
@@ -252,7 +252,7 @@ Example configuration::
         DIB_APT_LOCAL_CACHE: '0'
         DIB_DISABLE_APT_CLEANUP: '1'
         FS_TYPE: ext3
-    - name: ubuntu-xenial
+    - name: xenial
       pause: True
       rebuild-age: 86400
       formats: