[Plugins] Rename workload dammy.failure -> Dummy.failure

`dummy.failure' was the first class-based workload so
it got specific name started with lowercase `dummy'.
But other dummy scenarios have become class-based recently,
so dummy.failure is a kind of white crow now.

This patch simply renames the workload, without keeping
backward compatibility, because this is not critical.

Change-Id: I9ddeb0368adc7e517a74e6d25144b8e88ee4cd44

rally-jobs/rally.yaml

@@ -674,7 +674,7 @@
           action_3: 4.0
           action_4: 5.0
 
-  dummy.failure:
+  Dummy.failure:
     -
       args:
         sleep: 0.2

samples/tasks/scenarios/dummy/dummy-failure.json

@@ -1,5 +1,5 @@
 {
-    "dummy.failure": [
+    "Dummy.failure": [
         {
             "args": {
                 "sleep": 0.2,

samples/tasks/scenarios/dummy/dummy-failure.yaml

@@ -1,5 +1,5 @@
 ---
-  dummy.failure:
+  Dummy.failure:
     -
       args:
         sleep: 0.2