SRV/SRVSUB/PlatSrvSubscriptions.robot TC_MEC_SRV_SRVSUB_002_OK better set $SUBSCRIPTION_ID as golbal variable

better set $SUBSCRIPTION_ID as golbal variable, so it can be passed to below test cases as input, so that the test suite can run in a batch

suggestion: Check HTTP Response Header Contains Location Dictionary Should Contain Item ${response['body']} subscriptionType SerAvailabilityNotificationSubscription Dictionary Should Contain Item ${response['body']} subscriptionType SerAvailabilityNotificationSubscription Dictionary Should Contain Item ${response['body']} callbackReference ${APP_SRVSUB_NOTIF_CALLBACK_URI} Dictionary Should Contain Item ${response['body']} callbackReference ${APP_SRVSUB_NOTIF_CALLBACK_URI} ${SUBSCRIPTION_URL}= Get From Dictionary ${response['body']['_links']['self']} href ${SUBSCRIPTION_ID}= Fetch From Right ${SUBSCRIPTION_URL} / Set Global Variable ${SUBSCRIPTION_ID}