*: Support more role labels.

[runkecheng]

What type of PR is this?

/enhancement

Which issue(s) this PR fixes?

Fixes #330

What this PR does?

execute xenoncli raft disable in leader sample-mysql-2. then sample-mysql-2 become IDLE, Cluster re-electing.

xenoncli cluster status
+------------------------------------------+-------------------------------+--------+---------+--------------------------+---------------------+----------------+------------------------------------------+
|                    ID                    |             Raft              | Mysqld | Monitor |          Backup          |        Mysql        | IO/SQL_RUNNING |                 MyLeader                 |
+------------------------------------------+-------------------------------+--------+---------+--------------------------+---------------------+----------------+------------------------------------------+
| sample-mysql-2.sample-mysql.default:8801 | [ViewID:4 EpochID:2]@IDLE     | UNKNOW | OFF     | state:[NONE]␤            | [ALIVE] [READONLY]  | [true/true]    | sample-mysql-0.sample-mysql.default:8801 |
|                                          |                               |        |         | LastError:               |                     |                |                                          |
+------------------------------------------+-------------------------------+--------+---------+--------------------------+---------------------+----------------+------------------------------------------+
| sample-mysql-0.sample-mysql.default:8801 | [ViewID:4 EpochID:2]@LEADER   | UNKNOW | OFF     | state:[NONE]␤            | [ALIVE] [READWRITE] | [true/true]    | sample-mysql-0.sample-mysql.default:8801 |
|                                          |                               |        |         | LastError:               |                     |                |                                          |
+------------------------------------------+-------------------------------+--------+---------+--------------------------+---------------------+----------------+------------------------------------------+
| sample-mysql-1.sample-mysql.default:8801 | [ViewID:4 EpochID:2]@FOLLOWER | UNKNOW | OFF     | state:[NONE]␤            | [ALIVE] [READONLY]  | [true/true]    | sample-mysql-0.sample-mysql.default:8801 |
|                                          |                               |        |         | LastError:               |                     |                |                                          |
+------------------------------------------+-------------------------------+--------+---------+--------------------------+---------------------+----------------+------------------------------------------+

You can see that the role tag has been set to a corresponding role.

# all pods
kubectl get pod -o wide | grep sample
sample-mysql-0                         3/3     Running   0          40m     10.233.96.114   node2    <none>           <none>
sample-mysql-1                         3/3     Running   0          39m     10.233.90.108   node1    <none>           <none>
sample-mysql-2                         3/3     Running   0          37m     10.233.70.59    master   <none>           <none>
# leader
kubectl get -l role=LEADER pod
NAME             READY   STATUS    RESTARTS   AGE
sample-mysql-0   3/3     Running   0          30m
# follower
kubectl get -l role=FOLLOWER pod
NAME             READY   STATUS    RESTARTS   AGE
sample-mysql-1   3/3     Running   0          29m
# idle (disable raft)
kubectl get -l role=IDLE pod
NAME             READY   STATUS    RESTARTS   AGE
sample-mysql-2   3/3     Running   0          28m

kubectl describe svc sample-mysql
Name:              sample-mysql
...
Port:              mysql  3306/TCP
TargetPort:        3306/TCP
Endpoints:         10.233.70.59:3306,10.233.90.108:3306,10.233.96.114:3306
...

kubectl describe svc sample-follower
Name:              sample-follower
...
Port:              mysql  3306/TCP
TargetPort:        3306/TCP
Endpoints:         10.233.90.108:3306
...

kubectl describe svc sample-leader
Name:              sample-leader
...
Port:              mysql  3306/TCP
TargetPort:        3306/TCP
Endpoints:         10.233.96.114:3306
...

Special notes for your reviewer?

[acekingke]

LGTM

[andyli029]

LGTM.