add proxy support for RedHat

When install CentOS 7 required external access to *.fedoraproject.org.
If someone behind proxy enviroment, It block for a longtime..

This patch.. add proxy element to tdl, and add proxy settings to initrd.
```
<template>
   <name>oz-centos7</name>
   <description>CentOS 7 x86_64 template</description>
   <proxy>http://proxy.yourcompany.com:8088</proxy>
   <os>
      <name>CentOS-7</name>
      <version>7</version>
      <arch>x86_64</arch>
   </os>
</template>
```