bug: sofarpc-saga-sample start SofaRpc provider error

SofaRpcSagaProviderApplication.java start error:

```java
2020-10-29 16:39:05.120 ERROR --- [ main] o.s.boot.SpringApplication : Application startup failed
==>
org.springframework.context.ApplicationContextException: Unable to start embedded container; nested exception is org.springframework.context.ApplicationContextException: Unable to start EmbeddedWebApplicationContext due to missing EmbeddedServletContainerFactory bean.
at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.onRefresh(EmbeddedWebApplicationContext.java:137)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537)
at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693)
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:360)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:303)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107)
```

Start with SofaRpcSagaProviderApplication.java and reproduce its startup failure. Read the surrounding application configuration and dependencies to identify why the embedded servlet container factory is missing. Done means the SofaRpc provider sample starts without the reported ApplicationContextException.

Written by the indexing model from the issue text.