Werk #11540: Nutanix Prism Agent/prism_alerts check: add capability to filter for Prism Central

In case a Nutanix Prism Cluster was configured to use the alert check ({prism_alerts}) an alert addressing a certain VM could have been reported twice since it would have also been reported with the Prism Central instance (which would then always have the worst state of all monitored VMs). This change introduces a check option {prism_central_only} which set to True would consider only alerts addressing Prism Central VM (set in {vm_type} element of the alert context).

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