Change from manual URL construction to using a lib

Makes the code cleaner to read and more resilient to changes

Fixes https://jira.coreos.com/browse/QUAY-940

oauth/base.py

@@ -25,14 +25,7 @@ class OAuthEndpoint(object):
     params_copy = copy.copy(self.params)
     params_copy.update(parameters)
     return OAuthEndpoint(self.base_url, params_copy)
-
-  def to_url_prefix(self):
-    prefix = self.to_url()
-    if self.params:
-      return prefix + '&'
-    else:
-      return prefix + '?'
-
+    
   def to_url(self):
     (scheme, netloc, path, _, fragment) = urlparse.urlsplit(self.base_url)
     updated_query = urllib.urlencode(self.params)