Minor bugfix to secrets --file args read

Example steps to reproduce the error before fix:
```
$ echo -n 'bla' | kapitan secrets --write gpg:secrets/common/test_targets/gcp_project_id -f - -t test_terraform_dummy
Traceback (most recent call last):
  File "/Users/rjmco/.pyenv/versions/3.7.0/bin/kapitan", line 11, in <module>
    load_entry_point('kapitan==0.18.2', 'console_scripts', 'kapitan')()
  File "/Users/rjmco/.pyenv/versions/3.7.0/lib/python3.7/site-packages/kapitan/cli.py", line 279, in main
    secret_write(args, ref_controller)
  File "/Users/rjmco/.pyenv/versions/3.7.0/lib/python3.7/site-packages/kapitan/cli.py", line 291, in secret_write
    file_name = args.file_name
AttributeError: 'Namespace' object has no attribute 'file_name'
```

kapitan/cli.py

@@ -288,7 +288,7 @@ def main():
 def secret_write(args, ref_controller):
     "Write secret to ref_controller based on cli args"
     token_name = args.write
-    file_name = args.file_name
+    file_name = args.file
     data = None
 
     if file_name is None: