When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

@Deprecated
    public List<ProviderConfig> getProviders() {
        return convertProtocolToProvider(protocols);
    }
    /**
     * @deprecated Replace to setProtocols()
     */
    @Deprecated
    public void setProviders(List<ProviderConfig> providers) {
        this.protocols = convertProviderToProtocol(providers);
    }
    @Parameter(excluded = true)
    public String getUniqueServiceName() {
        StringBuilder buf = new StringBuilder();
        if (group != null && group.length() > 0) {
            buf.append(group).append("/");
        }
        buf.append(StringUtils.isNotEmpty(path) ? path : interfaceName);
        if (version != null && version.length() > 0) {
            buf.append(":").append(version);
        }
        return buf.toString();
    }
    @Override
    @Parameter(excluded = true)
    public String getPrefix() {
        return Constants.DUBBO + ".service." + interfaceName;
    }
}