Set env variable ELASTIC_NETINFO:false in Kibana (elastic#166156)

## Summary

This PR add the environmental veriable ELASTIC_NETINFO in the managed
and standalone manifests of Elasitc agent.

The variable has been introduced here
elastic/elastic-agent#3354

The reason for the introduction of the new variable
ELASTIC_NETINFO:false by default in the manifests, is related with the
work done elastic/integrations#6674

x-pack/plugins/fleet/server/services/elastic_agent_manifest.ts

@@ -68,6 +68,10 @@ spec:
                   fieldPath: metadata.name
             - name: STATE_PATH
               value: "/etc/elastic-agent"
+            # The following ELASTIC_NETINFO:false variable will disable the netinfo.enabled option of add-host-metadata processor. This will remove fields host.ip and host.mac.  
+            # For more info: https://www.elastic.co/guide/en/beats/metricbeat/current/add-host-metadata.html
+            - name: ELASTIC_NETINFO
+              value: "false"            
           securityContext:
             runAsUser: 0
             # The following capabilities are needed for 'Defend for containers' integration (cloud-defend)
@@ -389,6 +393,10 @@ spec:
               valueFrom:
                 fieldRef:
                   fieldPath: metadata.name
+            # The following ELASTIC_NETINFO:false variable will disable the netinfo.enabled option of add-host-metadata processor. This will remove fields host.ip and host.mac.  
+            # For more info: https://www.elastic.co/guide/en/beats/metricbeat/current/add-host-metadata.html
+            - name: ELASTIC_NETINFO
+              value: "false"             
           securityContext:
             runAsUser: 0
             # The following capabilities are needed for 'Defend for containers' integration (cloud-defend)