socket.close should be called directly

[Takanori Miyagishi]

The "disconnect" method is defined at common/ovsdb/base_connection and the method calls socket.close() only.

However socket.close() returns None regardless connection is established or not. Therefore, the method is actually meaningless. socket.close should be called directly

The following codes and result shows socket.close finished without any exceptions when socket is not established:

- Code
```
import socket

testsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
    res = testsocket.close()
except:
    print "exception is occurred."
else:
    print "exception is not occurred."
```

- Result
$ python2.7 ./socket_test.py
exception is not occurred.