Adds timestamps to scenarios iterations

In preparation for parallel scenario runners

Change-Id: I27fbaa2a546f47b2e00dc8e54348ec45410865a6

tests/unit/fakes.py

@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import datetime
 import multiprocessing
 import random
 import re
@@ -1314,6 +1315,9 @@ class FakeTimer(rally_utils.Timer):
     def duration(self):
         return 10
 
+    def timestamp(self):
+        return datetime.datetime(2015, 1, 1)
+
 
 @base_ctx.context("fake", order=1)
 class FakeContext(base_ctx.Context):