make metadata endpoints configurable as others

Rather have a working metadata exposure endpoint registration, whatever base url is,
than try to deduce it from entityID.

example/plugins/backends/saml2_backend.yaml.example

@@ -66,6 +66,8 @@ config:
           - [<name>/acs/post, 'urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST']
           discovery_response:
           - [<name>/disco, 'urn:oasis:names:tc:SAML:profiles:SSO:idp-discovery-protocol']
+	  metadata_exposal: <name>/proxy_saml2_backend.xml
+	  metadata_reload: <name>/reload-metadata
 
         # name_id_format: a list of strings to set the <NameIDFormat> element in SP metadata
         # name_id_policy_format: a string to set the Format attribute in the NameIDPolicy element

src/satosa/backends/saml2.py

@@ -611,14 +611,12 @@ def register_endpoints(self):
                     ("^%s$" % endp, self.disco_response))
 
         if self.expose_entityid_endpoint():
-            logger.debug("Exposing backend entity endpoint = {}".format(self.sp.config.entityid))
-            parsed_entity_id = urlparse(self.sp.config.entityid)
-            url_map.append(("^{0}".format(parsed_entity_id.path[1:]),
-                            self._metadata_endpoint))
+            url_map.append(
+                ("^%s$" % sp_endpoints["metadata_exposal"], self._metadata_endpoint))
 
         if self.enable_metadata_reload():
             url_map.append(
-                ("^%s/%s$" % (self.name, "reload-metadata"), self._reload_metadata))
+                ("^%s$" % sp_endpoints["metadata_reload"], self._reload_metadata))
 
         logger.debug(f"Loaded SAML2 endpoints: {url_map}")
         return url_map