NLU server logging to files does not work
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**Rasa NLU version**: all

**Operating system** (windows, osx, ...): OSX


**Issue**:

Logging to files with `--write` option doesn't work. Replacing the `_configure_logging()` method in `rasa_nlu.server` fixes the problem:
```python
@staticmethod
    def _configure_logging(loglevel, logfile):
        for handler in logging.root.handlers[:]:
            logging.root.removeHandler(handler)
        logging.basicConfig(filename=logfile,
                            level=loglevel)
        logging.captureWarnings(True)
```
I'm not sure if there's a better approach.

Also, there is no command line option for `--loglevel` which that function is supposed to use.

diff --git a/rasa/nlu/server.py b/rasa/nlu/server.py
index 49c89c2003f..4f35a543a1b 100644
--- a/rasa/nlu/server.py
+++ b/rasa/nlu/server.py
@@ -143,8 +143,11 @@ class RasaNLU(object):
 
     @staticmethod
     def _configure_logging(loglevel, logfile):
-        logging.basicConfig(filename=logfile,
-                            level=loglevel)
+        if logfile is None:
+            utils.configure_colored_logging(loglevel)
+        else:
+            logging.basicConfig(filename=logfile,
+                                level=loglevel)
         logging.captureWarnings(True)
 
     @app.route("/", methods=['GET', 'OPTIONS'])
@@ -362,7 +365,6 @@ def get_token(_clitoken: str) -> str:
 
 
 def main(args):
-    utils.configure_colored_logging(args.loglevel)
     pre_load = args.pre_load
 
     _endpoints = read_endpoints(args.endpoints)
['rasa/nlu/server.py']