RHEL6: yum command reports "Error: No module named yum" message

Issue

• When executing yum clean all or any other yum commland the following error is seen :

  'import site' failed; use -v for traceback
  There was a problem importing one of the Python modules
  required to run yum. The error leading to this problem was:
  
  No module named yum
  
  Please install a package which provides this module, or
  verify that the module is installed correctly.