allow pre-loading API tokens from config

This is the first small part of easing the pain of services,
which is generating the API tokens,
and used to require initializing the JupyterHub database.

jupyterhub/orm.py

@@ -303,11 +303,19 @@ class User(Base):
                 name=self.name,
             )
 
-    def new_api_token(self):
-        """Create a new API token"""
+    def new_api_token(self, token=None):
+        """Create a new API token
+        
+        If `token` is given, load that token.
+        """
         assert self.id is not None
         db = inspect(self).session
-        token = new_token()
+        if token is None:
+            token = new_token()
+        else:
+            found = APIToken.find(db, token)
+            if found:
+                raise ValueError("Collision on token: %s..." % token[:4])
         orm_token = APIToken(user_id=self.id)
         orm_token.token = token
         db.add(orm_token)