Fix keepcache values of yum_repository (#7506)

As the official document[1], the parameter keepcache should be
'0' or '1' as string. To avoid the following warning message,
this fixes the parameter value:

  [WARNING]: The value False (type bool) in a string field was
  converted to u'False' (type string). If this does not look
  like what you expect, quote the entire value to ensure it
  does not change.

https://docs.ansible.com/ansible/latest/collections/ansible/builtin/yum_repository_module.html