Fix identifier name problem

* Expected: /Users/xxx/.vagrant.d/insecure_private_key
* Actual: [/Users/xxx/.vagrant.d/insecure_private_key]

Please see below, and let me contribute the problem fix.

```
$ vagrant provision
[default] Running provisioner: fabric...
[127.0.0.1] Executing task 'install'
[127.0.0.1] run: ls -la
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/main.py", line 743, in main
    *args, **kwargs
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/tasks.py", line 368, in execute
    multiprocessing
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/tasks.py", line 264, in _execute
    return task.run(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/tasks.py", line 171, in run
    return self.wrapped(*args, **kwargs)
  File "/Users/kch/Vagrant/sucho/fabfile.py", line 11, in install
    run('ls -la')
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/network.py", line 578, in host_prompting_wrapper
    return func(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/operations.py", line 1042, in run
    shell_escape=shell_escape)
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/operations.py", line 909, in _run_command
    channel=default_channel(), command=wrapped_command, pty=pty,
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/state.py", line 388, in default_channel
    chan = _open_session()
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/state.py", line 380, in _open_session
    return connections[env.host_string].get_transport().open_session()
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/network.py", line 118, in __getitem__
    self.connect(key)
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/network.py", line 110, in connect
    self[key] = connect(user, host, port, sock)
  File "/Library/Python/2.7/site-packages/Fabric-1.8.0-py2.7.egg/fabric/network.py", line 392, in connect
    sock=sock
  File "build/bdist.macosx-10.7-intel/egg/paramiko/client.py", line 342, in connect
  File "build/bdist.macosx-10.7-intel/egg/paramiko/client.py", line 462, in _auth
  File "build/bdist.macosx-10.7-intel/egg/paramiko/pkey.py", line 198, in from_private_key_file
  File "build/bdist.macosx-10.7-intel/egg/paramiko/rsakey.py", line 51, in __init__
  File "build/bdist.macosx-10.7-intel/egg/paramiko/rsakey.py", line 163, in _from_private_key_file
  File "build/bdist.macosx-10.7-intel/egg/paramiko/pkey.py", line 279, in _read_private_key_file
IOError: [Errno 2] No such file or directory: '[/Users/xxx/.vagrant.d/insecure_private_key]'
```