intel_pmu plugin: implement core groups option

intel_pmu plugin: implement core groups option

Having this option will allow to configure monitoring of PMU
events only on specific CPUs thus decrease number of opened
descriptors significantly and avoid 'Too many open files' errors.
New "Cores" option is similar to that available in intel_rdt plugin.
New tool utils_config_cores is added to parse "Cores" option.

Change-Id: I8f792e1f2560c4cf19aee101fdb07c925d682778
Signed-off-by: Kamil Wiatrowski <kamilx.wiatrowski@intel.com>