enh(emc/isilon): add cluster cpu mode

[cgagnaire]

Add a new mode to EMC Isilon plugin to check cluster CPU usage.

perl centreon_plugins.pl --plugin=storage::emc::isilon::snmp::plugin --mode=cluster-cpu --hostname='10.1.2.3' --critical-idle=10:
OK: CPU Usage: User 20.40 %, Nice 0.00 %, System 44.10 %, Idle 32.30 %, Interrupt 3.20 % | 'cpu.user.utilization.percentage'=20.40%;;;0;100 'cpu.nice.utilization.percentage'=0.00%;;;0;100 'cpu.system.utilization.percentage'=44.10%;;;0;100 'cpu.idle.utilization.percentage'=32.30%;;10:;0;100 'cpu.interrupt.utilization.percentage'=3.20%;;;0;100