Add Minimum and Maximum variables to the Agent CPU Load check

[kbilev]

Is your feature request related to a problem? Please describe.
Performance Data only provides the warning, critical and min=0 values, no maximum value.
When using for example Tachographs in a Dashboard with CPU Load provided by the agent, the minimum is set to 0 and the maximum to 100.

Describe the solution you'd like
Add arguments for Minimum and Maximum to the CPU load check (and if they are empty, continue to use the default values)

Describe alternatives you've considered
As workaround I have added in the SystemLoad.php a $critical + 10 as maximum value which is already better than the default empty value or 100 in the Tachograph as it is empty.

Additional context
Before:

After:

[kbilev]

The workaround

$this->addPerfdata(new PerfdataGauge(
            'load1',
            round($this->json['system_load'][0], 2),
            '',
            $warning,
            $critical,
            0,
            $critical + 10
        ));

        $this->addPerfdata(new PerfdataGauge(
            'load5',
            round($this->json['system_load'][1], 2),
            '',
            $warning,
            $critical,
            0,
            $critical + 10
        ));

        $this->addPerfdata(new PerfdataGauge(
            'load15',
            round($this->json['system_load'][2], 2),
            '',
            $warning,
            $critical,
            0,
            $critical + 10
        ));