Use urljoin() to compose OAuth URLs instead of string concatenation

String concatenation is bound to mess up; in this case it was producing
double slashes instead of single ones when `BLENDER_ID_ENDPOINT` ends in
a slash. Since URLs generally end in a slash, this should be supported.

pillar/config.py

@@ -32,7 +32,7 @@ SECRET_KEY = ''
 AUTH_TOKEN_HMAC_KEY = b''
 
 # Authentication settings
-BLENDER_ID_ENDPOINT = 'http://id.local:8000'
+BLENDER_ID_ENDPOINT = 'http://id.local:8000/'
 
 CDN_USE_URL_SIGNING = True
 CDN_SERVICE_DOMAIN_PROTOCOL = 'https'