An IP address is required for a zabbix host

[rnelson0]

In #133, the default $listenip value was removed. When this is not present, the following error is observed:

Error: Could not set 'present' on ensure: Server answer API error:
 {
  "code": -32602,
  "message": "Invalid params.",
  "data": "Incorrect arguments passed to function."
}
 on request:
 {
  "method": "host.create",
  "params": [
    {
      "host": "zabbix.nelson.va",
      "interfaces": [
        {
          "type": 1,
          "main": 1,
          "ip": null,
          "dns": "zabbix.nelson.va",
          "port": "10050",
          "useip": 1
        }
      ],
      "status": 0,
      "available": 1,
      "groups": [
        {
          "groupid": 2
        }
      ],
      "proxy_hostid": null,
      "templates": [
        10001,
        10102
      ]
    }
  ],
  "id": 8903,
  "jsonrpc": "2.0",
  "auth": "752a36e6c48997194ee8b7ecb813fc40"
} at 27:/etc/puppetlabs/code/environments/zabbix/modules/zabbix/manifests/resources/agent.pp

By replacing the else{} block, this functions properly. PR forthcoming.