Fix shunning reader when node goes to MAINTENANCE

[alpes214]

Description:
This PR fixes issue #2657

Testing:

1. Configuring mysql_galera_hostgroups

MySQL [(none)]> select * from mysql_galera_hostgroups\G
*************************** 1. row ***************************
       writer_hostgroup: 1
backup_writer_hostgroup: 2
       reader_hostgroup: 3
      offline_hostgroup: 4
                 active: 1
            max_writers: 1
  writer_is_also_reader: 1
max_transactions_behind: 150
                comment: NULL
1 row in set (0.00 sec)

2. Initial state of the runtime_mysql_servers

MySQL [(none)]> select * from runtime_mysql_servers;
+--------------+-------------+------+-----------+--------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
| hostgroup_id | hostname    | port | gtid_port | status | weight | compression | max_connections | max_replication_lag | use_ssl | max_latency_ms | comment |
+--------------+-------------+------+-----------+--------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
| 1            | 172.18.0.30 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 2            | 172.18.0.20 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 2            | 172.18.0.10 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.10 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.30 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.20 | 3306 | 0         | ONLINE | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
+--------------+-------------+------+-----------+--------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
6 rows in set (0.00 sec)

3. After setting set global pxc_maint_mode=MAINTENANCE; on a writer's node

MySQL [(none)]> select * from runtime_mysql_servers;
+--------------+-------------+------+-----------+---------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
| hostgroup_id | hostname    | port | gtid_port | status  | weight | compression | max_connections | max_replication_lag | use_ssl | max_latency_ms | comment |
+--------------+-------------+------+-----------+---------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
| 1            | 172.18.0.30 | 3306 | 0         | SHUNNED | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 2            | 172.18.0.10 | 3306 | 0         | ONLINE  | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.10 | 3306 | 0         | ONLINE  | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.30 | 3306 | 0         | SHUNNED | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 3            | 172.18.0.20 | 3306 | 0         | ONLINE  | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
| 1            | 172.18.0.20 | 3306 | 0         | ONLINE  | 1      | 0           | 1000            | 0                   | 0       | 0              |         |
+--------------+-------------+------+-----------+---------+--------+-------------+-----------------+---------------------+---------+----------------+---------+
6 rows in set (0.00 sec)

[renecannao]

Cherry-picked in 2.1.0