data/userconfig.yaml: Change default logging to print full shas at init and bst show

This may be obnoxious for `bst show` purposes, which will now also print
the full artifact cache keys instead of nicely abbreviated ones, but it is a
more appropriate default for the heading of a build session.

This fixes issue #343

buildstream/data/userconfig.yaml

@@ -77,7 +77,7 @@ logging:
   # also determines the default display format for `bst show`
   element-format: |
 
-    %{state: >12} %{key} %{name} %{workspace-dirs}
+    %{state: >12} %{full-key} %{name} %{workspace-dirs}
 
   # Format string for all log messages.
   message-format: |