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: Ie81f13a96502236bdfa7fc537e8fa3a10481cf36
Bug-Url: https://bugzilla.redhat.com/1078897
Signed-off-by: Shirly Radco <sradco@redhat.com>