Switch task parser to custom yaml

1.Switch task parser to custom yaml, custom yaml deprecates duplicate key
and loads task in the same order as they are in task file.
2.Remove duplicate scenario KeystoneBasic.create_delete_user.

Change-Id: I3b338f63eca0ac217a6376f6d6705da69fb97e80

rally-jobs/rally-keystone-api-v2.yaml

@@ -197,17 +197,6 @@
         failure_rate:
           max: 0
 
-  KeystoneBasic.create_delete_user:
-    -
-      args: {}
-      runner:
-        type: "constant"
-        times: 10
-        concurrency: 10
-      sla:
-        failure_rate:
-          max: 0
-
   KeystoneBasic.create_and_delete_service:
     -
       runner:

rally-jobs/rally-mos.yaml

@@ -116,17 +116,6 @@
         failure_rate:
           max: 0
 
-  KeystoneBasic.create_delete_user:
-    -
-      args: {}
-      runner:
-        type: "constant"
-        times: 1
-        concurrency: 1
-      sla:
-        failure_rate:
-          max: 0
-
   HeatStacks.create_and_list_stack:
     -
       args:

rally-jobs/rally.yaml

@@ -197,17 +197,6 @@
         failure_rate:
           max: 0
 
-  KeystoneBasic.create_delete_user:
-    -
-      args: {}
-      runner:
-        type: "constant"
-        times: 10
-        concurrency: 10
-      sla:
-        failure_rate:
-          max: 0
-
   KeystoneBasic.create_and_delete_service:
     -
       runner: