ShardingRule does not verify the existence of the data source, at the startup stage

## Bug Report

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### Which version of ShardingSphere did you use?
master b90e5a17

### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?
ShardingSphere-Proxy

### Expected behavior
I use config like this
```yaml
databaseName: sharding_db

dataSources:
ds_0:
xxx
ds_1:
xxx

rules:
- !SHARDING
defaultDatabaseStrategy:
standard:
shardingAlgorithmName: database_inline
shardingColumn: user_id
shardingAlgorithms:
database_inline:
props:
algorithm-expression: readwrite_ds
type: INLINE
t_order_inline:
props:
algorithm-expression: t_order_$->{order_id % 2}
type: INLINE
tables:
t_order:
actualDataNodes: readwrite_ds.t_order_$->{0..1}
logicTable: t_order
tableStrategy:
standard:
shardingAlgorithmName: t_order_inline
shardingColumn: order_id
```

When initializing the ShardingRule, it should be checked that the database readwrite_ds does not exist.
### Actual behavior
Proxy start successful

### Reason analyze (If you can)
missing check