Add "description" to task config

Fetch "description" from scenario doc string if no user specified
description then add it to the task config. String "description"
property will be added to the config.

Partial-bug: #1447496

Co-Authored-By: Andrey Kurilin <andr.kurilin@gmail.com>

Change-Id: I882938e86d6c8209b669d7220285e8aa05bdd3e3

rally-jobs/rally.yaml

@@ -298,6 +298,7 @@
           max_degradation: 50
 
     -
+      description: "Check 'constant' runner."
       args:
         sleep: 0.25
       runner:
@@ -310,6 +311,7 @@
           max: 0
 
     -
+      description: "Check 'constant_for_duration' runner."
       args:
         sleep: 0.1
       runner:
@@ -321,6 +323,7 @@
           max: 0
 
     -
+      description: "Check 'rps' runner."
       args:
         sleep: 0.001
       runner:
@@ -332,6 +335,7 @@
           max: 0
 
     -
+      description: "Check 'rps' runner with float value of requests per second."
       args:
         sleep: 0.1
       runner:
@@ -354,6 +358,7 @@
           max: 0
 
     -
+      description: "Check 'max_concurrency' and 'max_cpu_count' properties of 'rps' runner."
       args:
         sleep: 0.001
       runner:
@@ -367,6 +372,7 @@
           max: 0
 
     -
+      description: "Check 'serial' runner."
       args:
         sleep: 0.1
       runner:
@@ -390,6 +396,7 @@
         failure_rate:
           max: 0
     -
+      description: "Check 'quotas' context."
       args:
         sleep: 0.01
       runner: