drush installing drupal through root ssh is causing 500 internal server error
Thanks to Matt C from Known Host who helped me resolving this issue. Sharing his reply below so others can be benefited.


  I've run the fix permissions script and it looks like all files and
  directories are set as they should be. You can find the script in
  /public_html/fixperms.sh. I've also provided you with a link for
  fixpermissions:
  
  http://boomshadow.net/tech/fixes/fixperms-script/