Improvement for better identification of RHEL distros

RHEL and RHEL based distros contain file or symlink /etc/redhat-release, so it's possible to identify it using:
```
if os.path.isfile('/etc/redhat-release'):
```
Tested on CentOS, RHEL and Fedora:
```
$ python
Python 2.6.6 (r266:84292, Jan 22 2014, 09:42:36) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.path.isfile('/etc/redhat-release')
True
>>> 
```