sleeptest: Modify to make use of self.params/multiplex file

Modify sleeptest to pick any params from self.params.
Also, introduce a sample multiplex file, sleeptest.mplx,
an example of how to write a multiplex file to fulfill
matrix defining goals.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

tests/sleeptest/sleeptest.mplx

+variants:
+    - sleeptest:
+        variants:
+            - short:
+                sleep_length = 0.5
+            - medium:
+                sleep_length = 1
+            - long:
+                sleep_length = 5

tests/sleeptest/sleeptest.py

@@ -27,10 +27,11 @@ class sleeptest(test.Test):
     Example test for avocado.
     """
 
-    def action(self, length=1):
+    def action(self):
         """
         Sleep for length seconds.
         """
+        length = float(self.params.get('sleep_length', 1))
         self.log.debug("Sleeping for %d seconds", length)
         time.sleep(length)