Include database system identifier in cluster info

patroni/api.py

@@ -278,6 +278,7 @@ class RestApiHandler(BaseHTTPRequestHandler):
                 'postmaster_start_time': row[0],
                 'role': 'replica' if row[1] else 'master',
                 'server_version': self.server.patroni.postgresql.server_version,
+                'database_system_identifier': self.server.patroni.postgresql.sysid,
                 'xlog': ({
                     'received_location': row[3],
                     'replayed_location': row[4],

tests/test_api.py

@@ -17,6 +17,7 @@ class MockPostgresql(object):
     state = 'running'
     role = 'master'
     server_version = '999999'
+    sysid = 'dummysysid'
     scope = 'dummy'
 
     @staticmethod