Make self-rally more stable

Even if we have failure probability 0.5 it may occure that failure rate
will be any particual number (hello to the math) so we should extend
min and max failure rates from 20 to 80 to 5 to 95 to reduce the possibility
of such event

Change-Id: Id5c79408bbe6a0a1850c386bc3d4710d03a0512c

rally-jobs/self-rally.yaml

@@ -366,8 +366,8 @@
               times: 2042
           sla:
             failure_rate:
-              min: 20
-              max: 80
+              min: 5
+              max: 95
         -
           scenario:
             Dummy.dummy_output: {}
@@ -384,8 +384,8 @@
               exception_probability: 0.5
           sla:
             failure_rate:
-              min: 20
-              max: 80
+              min: 5
+              max: 95
           runner:
             constant:
               times: 50