Able to configure SNMP manager with invalid port (Ex: alpha & negative port)

[gtmills]

Able to configure SNMP manager with invalid port (Ex: alpha & negative port)
Internal defect number: 525716
Steps to create:

curl -k -H "X-Auth-Token: $bmc_token" -X POST -D headers.txt https://${bmc}/redfish/v1/EventService/Subscriptions -d '{"Destination":"snmp://xx.yy.xx.yy:ab", "SubscriptionType":"SNMPTrap", "Protocol":"SNMPv2c"}'
{
  [
    {
      "@odata.type": "#Message.v1_1_1.Message",
      "Message": "The resource has been created successfully.",
      "MessageArgs": [],
      "MessageId": "Base.1.13.0.Created",
      "MessageSeverity": "OK",
      "Resolution": "None."
    }
  ]
}
bash-4.2$ curl -k -H "X-Auth-Token: $bmc_token" -X POST -D headers.txt https://${bmc}/redfish/v1/EventService/Subscriptions -d '{"Destination":"snmp://xx.yy.xx.yy:-40", "SubscriptionType":"SNMPTrap", "Protocol":"SNMPv2c"}'
{
   [
    {
      "@odata.type": "#Message.v1_1_1.Message",
      "Message": "The resource has been created successfully.",
      "MessageArgs": [],
      "MessageId": "Base.1.13.0.Created",
      "MessageSeverity": "OK",
      "Resolution": "None."
    }
  ]
}

[ChicagoDuan]

Done, please review: ibm-openbmc/bmcweb#688

[ChicagoDuan]

Merged: ibm-openbmc/bmcweb#688