Improve meta-service config usability

[drmingdrmer]

Currently, the first meta-service node in a cluster must be configured with single, and others are configured with join:

# node-1
[raft_config]
id = 1
single        = true

# node-2
[raft_config]
id            = 2
join          = ["127.0.0.1:9001"]

This single/join mechanism introduces complexity setting up a cluster.
It is possible to specify the cluster configure with one same config for every node:

# node-1
[raft_config]
id = 1
cluster = ["127.0.0.1:9001", "127.0.0.1:9002"]

# node-2
[raft_config]
id            = 2
cluster = ["127.0.0.1:9001", "127.0.0.1:9002"]