Add connect_timeout option for LDAP connection

That option will enable the LDAP client to give up
to connect to the server in the specific seconds.

ldapauthenticator/ldapauthenticator.py

@@ -236,6 +236,13 @@ def _server_port_default(self):
         primary LDAP server is unavailable.
         """,
     )
+    connect_timeout = Int(
+        config=True,
+        default=15,
+        help="""
+        LDAP client connect timeout (seconds)
+        """,
+    )
 
     def resolve_username(self, username_supplied_by_user):
         search_dn = self.lookup_dn_search_user
@@ -337,7 +344,10 @@ def get_connection(self, userdn, password):
 
     def _get_real_connection(self, userdn, password, server_address, server_port):
         server = ldap3.Server(
-            server_address, port=server_port, use_ssl=self.use_ssl
+            server_address,
+            port=server_port,
+            use_ssl=self.use_ssl,
+            connect_timeout=self.connect_timeout,
         )
         auto_bind = (
             ldap3.AUTO_BIND_NO_TLS if self.use_ssl else ldap3.AUTO_BIND_TLS_BEFORE_BIND