Added functional tests for VNF Performance Management

...03/VNFPerformanceManagement-API_nxw/IndividualPmJob.robot

...erformanceManagement-API_nxw/IndividualSubscription.robot

...NFPerformanceManagement-API_nxw/IndividualThreshold.robot

SOL003/VNFPerformanceManagement-API_nxw/PMJobs.robot

SOL003/VNFPerformanceManagement-API_nxw/Subscriptions.robot

.../VNFPerformanceManagement-API_nxw/environment/generic.txt

...Management-API_nxw/environment/individualSubscription.txt

...ceManagement-API_nxw/environment/individualThresholds.txt

...rformanceManagement-API_nxw/environment/subscriptions.txt

...VNFPerformanceManagement-API_nxw/jsons/subscriptions.json

...nceManagement-API_nxw/schemas/PmSubscriptions.schema.json

...manceManagement-API_nxw/schemas/Subscriptions.schema.json