NetworkMonitoring missed abstract method.

Report came out when nwmongw ran with zabbix base driver.

```
I, [2013-08-10T20:22:45.794310 #33512]  INFO -- MonitoringGateway: Refreshing monitoring configuration for i-1ieyj8b6
2013-08-10 20:22:45 ThreadPool thr=#<Thread:0x007f6fd06a6bb0> [ERROR]: Caught NoMethodError: undefined method `update_instance' for #<Dcmgr::Drivers::Zabbix:0x007f6fd06a6980>
        ./bin/nwmongw:51:in `refresh_monitoring'
        ./bin/nwmongw:35:in `block (3 levels) in <class:MonitoringGateway>'
        /opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/isono-0.2.19/lib/isono/thread_pool.rb:32:in `call'
        /opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/isono-0.2.19/lib/isono/thread_pool.rb:32:in `block (2 levels) in initialize'
```