don't parse subdirs in host/group_vars

as those are not supported in
core vars_plugins/group_vars.py
but might be used by other vars_plugins
Fix grammatical error in error message
bug fix in first_found search path assembly
Merge pull request #2903 from sergevanginderachter/first_found

bug fix in first_found search path assembly
Merge pull request #2902 from timhabermaas/fix-grammatical-error

Fix grammatical error in error message
Merge pull request #2899 from sergevanginderachter/host-group-vars-no-subdirs

don't parse subdirs in host/group_vars
Fix listification inside of with_items so it runs more intelligently.
Squashed commit of the following:

commit 08fd4a1afca28c0ad58fe791e943738cb08bd614
Author: Ton Kersten <github@tonkersten.com>
Date:   Tue May 14 12:50:39 2013 +0200

    Check if the 'holders' directory exists. If not, skip.