Bug fix: don't fall back to user/pass if kerberos is configured

this was revealed by the recently added integration test:
https://github.com/kiwitcms/tcms-api/runs/522277212

the root cause is that if statement checking for `use_kerberos`
doesn't have a false branch and execution falls-through to
raising exception b/c username/password aren't defined in tcms.conf

tcms_api/__init__.py

@@ -97,15 +97,18 @@ def __init__(self):
         if strtobool(config['tcms'].get('use_kerberos', 'False')):
             # use Kerberos
             TCMS._connection = TCMSKerbXmlrpc(config['tcms']['url']).server
+            return
 
-        # use plain authentication otherwise
         try:
+            # use password authentication
             TCMS._connection = TCMSXmlrpc(config['tcms']['username'],
                                           config['tcms']['password'],
                                           config['tcms']['url']).server
         except KeyError:
             raise Exception("username/password required in %s" % self._path)
 
+        return
+
     @property
     def exec(self):
         """