history: Updated calculation of cpu usage

Updated calculations of system_cpu_usage_percent
and user_cpu_usage_percent according to the
number of cpus.

Change-Id: Idfae05895d8450e6d5f450fbf2a9e934e65a5785
Bug-Url: https://bugzilla.redhat.com/1078897
Signed-off-by: Shirly Radco <sradco@redhat.com>