This adds SLA plugin that finds minimum and maximum duration of
iterations completed without errors during Rally task execution.
Assuming that minimum duration is 100%, it calculates
performance degradation against maximum duration.
Example config:
sla:
performance_degradation:
max_degradation: 75
Spec: sla_pd_plugin
Change-Id: Ieedba7be72364f5599a3c0cf79f5f494a7391ea0