Fix short_description flag in plugins, role modules and templates

Before "short description" was used in most plugins, modules and also
in the new module templates.

ansible-doc was therefore not showing the short description. To fix the
issue the flag was renamed to short_description instead.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2121362
       'ansible-doc' -l lists most idm modules as 'UNDOCUMENTED'