slapos_panel: do not build url by joing strings

master/bt5/slapos_panel/SkinTemplateItem/portal_skins/slapos_panel/Project_addSlapOSComputeNodeToken.py

+from urlparse import urljoin
+
 portal = context.getPortalObject()
 person = portal.portal_membership.getAuthenticatedMember().getUserValue()
 
@@ -26,7 +28,7 @@ if slapos_master_web_url is None:
 
 # XXX this url is HARDCODED on the client side
 # and so, can not be modified
-request_url = "%s/%s" % (slapos_master_web_url, "Person_requestComputer")
+request_url = urljoin(slapos_master_web_url, "Person_requestComputer")
 
 person.requestToken(request_url=request_url)
 access_token_id = context.REQUEST.get("token")