Add dhcp server bundle and unit test

put global variable into class variable would be better.
```
class Dhcp(Sanji):
    path_root = os.path.abspath(os.path.dirname(__file__))
    dhcpd_config_path = "/etc/dhcp/dhcpd.conf"
    # retry times when model initialize
    retry_times = 5
```

otherwise, import file may overwritten your variable...