Apollo v1.7.2 release

This release includes the following fix.

• update xstream to 1.4.15 #3524

Installation

Please refer 分布式部署指南

How to upgrade from v1.7.1 to v1.7.2

There is no schema change between v1.7.1 and v1.7.2
So simply deploy v1.7.2 executables with the following sequences:

1. apollo-configservice
2. apollo-adminservice
3. apollo-portal