Render 'identity.authenticate' meter name with correct status

jsonpath-rw-ext uses operator.add for "+" and operator.sub for "-". operator.add can do string concatenation, but operator.sub cannot do string subtraction. so the original hack does not work, perhaps we should use sub to get an empty string.

Closes-Bug: 2032882

Change-Id: I9183a4a9f9cc345d8b4508bd7de1afa0eccd89d1
(cherry picked from commit 144827d304bbd7ffc9aa8165a4725d6fd00d73cc)
(cherry picked from commit a0c5638)

ceilometer/data/meters.d/meters.yaml

@@ -383,7 +383,7 @@ metric:
 
   # Identity
   # NOTE(gordc): hack because jsonpath-rw-ext can't concat starting with string.
-  - name: $.payload.outcome - $.payload.outcome + 'identity.authenticate.' + $.payload.outcome
+  - name: $.payload.outcome.`sub(/.*/, )` + 'identity.authenticate.' + $.payload.outcome
     type: 'delta'
     unit: 'user'
     volume: 1