Description of problem:

When using --installroot option to install package to a specific directory, such as /usr/local/nginx, yum always uses an incorrect URL to get repomd.xml: http://mirrors.163.com/centos/%24releasever. The correct one should be http://mirrors.163.com/centos/6.

The command also produces unrelated files in the intended installroot:

/usr/local/nginx
└── var
    ├── cache
    │   └── yum
    │       └── x86_64
    │           └── $releasever
    │               ├── base
    │               │   └── packages
    │               ├── epel
    │               │   ├── metalink.xml
    │               │   └── packages
    │               └── timedhosts.txt
    ├── lib
    │   ├── rpm
    │   │   ├── Name
    │   │   ├── Packages
    │   │   └── Providename
    │   └── yum
    │       ├── repos
    │       │   └── x86_64
    │       │       └── $releasever
    │       │           ├── base
    │       │           └── epel
    │       └── uuid
    ├── log
    │   └── yum.log
    └── run


Version-Release number of selected component (if applicable):
yum: 3.2.29

How reproducible:

# yum --installroot=/usr/local/nginx install nginx

Loaded plugins: fastestmirror, presto
Determining fastest mirrors
epel/metalink                                                                                                                                              | 4.5 kB     00:00     
Including mirror: mirror.neu.edu.cn
Including mirror: mirrors.ustc.edu.cn
 * epel: mirror.neu.edu.cn
http://mirrors.163.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404"
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again



Steps to Reproduce:
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Actual results:


Expected results:


Additional info: