[DUBBO-2.7.8]Fix DynamicConfiguration namespace property read bug when use custom namespace

[furaul]

Github issue

#6606

What is the purpose of the change

Fix ConfigCenter namespace read bug in apollo, etcd and so on.
修复apollo、etcd等配置中心读取namespace错误的bug。

Brief changelog

use org.apache.dubbo.common.constants.CommonConstants#CONFIG_NAMESPACE_KEY instead of org.apache.dubbo.common.config.configcenter.Constants#CONFIG_NAMESPACE_KEY.

在apollo、etcd等配置中心的实现中，使用org.apache.dubbo.common.constants.CommonConstants#CONFIG_NAMESPACE_KEY 来替换org.apache.dubbo.common.config.configcenter.Constants#CONFIG_NAMESPACE_KEY，确保自配置的配置中心namespace可以正确读取。

Verifying this change

All unit tests have passed.

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[furaul]

@beiwei30 @chickenlj

[AlbumenJ]

Good job

[kevinw66]

lgtm