sites/defalt/files need to be rewritable during configuration time.
   simply change write permission with a+w will not work on fedora 
	since SELinux is turned on.  
	we need to use chcon -R -t httpd_sys_rw_content_t files 
	to change the file type from httpd_sys_content_t to httpd_sys_rw_content_t
	Very ticky!!!

[root@class default]# ls -Z
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 default.settings.php
drwxr-xr-x. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 files
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_rw_content_t:s0 settings.php
[root@class default]# chmod a+w files
[root@class default]# ls -Z
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 default.settings.php
drwxrwxrwx. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 files
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_rw_content_t:s0 settings.php
[root@class default]# chcon -R -t httpd_sys_rw_content_t files
[root@class default]# ls -Z
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_content_t:s0 default.settings.php
drwxrwxrwx. apache apache unconfined_u:object_r:httpd_sys_rw_content_t:s0 files
-rw-rw-rw-. apache apache unconfined_u:object_r:httpd_sys_rw_content_t:s0 settings.php