Fix RuntimeError of lldp_packet_in_handler
Fix RuntimeError of lldp_packet_in_handler

Items should not be removed during iteration

Error message
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/ryu/base/app_manager.py", line 290, in _event_loop
    handler(ev)
  File "/usr/local/lib/python3.4/dist-packages/ryu/topology/switches.py", line 821, in lldp_packet_in_handler
    for host in self.hosts.values():
RuntimeError: dictionary changed size during iteration
```

Signed-off-by: Yi Tseng <769b085a2a82a4408df352b5e3ec66ebe856cd00@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

gareging/SDN_Framework,shinpeimuraoka/ryu,TakeshiTseng/ryu,fujita/ryu,iwaseyusuke/ryu,TakeshiTseng/ryu,fkakuma/ryu,iwaseyusuke/ryu,pichuang/ryu,lagopus/ryu-lagopus-ext,fujita/ryu,pichuang/ryu,fujita/ryu,gareging/SDN_Framework,osrg/ryu,fujita/ryu,fkakuma/ryu,lagopus/ryu-lagopus-ext,pichuang/ryu,gareging/SDN_Framework,fujita/ryu,osrg/ryu,lagopus/ryu-lagopus-ext,osrg/ryu,lagopus/ryu-lagopus-ext,fkakuma/ryu,shinpeimuraoka/ryu,iwaseyusuke/ryu,fkakuma/ryu,osrg/ryu,gareging/SDN_Framework,iwaseyusuke/ryu,shinpeimuraoka/ryu,lagopus/ryu-lagopus-ext,osrg/ryu,gareging/SDN_Framework,TakeshiTseng/ryu,TakeshiTseng/ryu,fkakuma/ryu,iwaseyusuke/ryu,pichuang/ryu,shinpeimuraoka/ryu,pichuang/ryu,shinpeimuraoka/ryu,TakeshiTseng/ryu