salt: make grain-ensure.py operate locally

This patch is meant to solve an issue with grain-ensure.py hanging when
salt-minion is unable to authenticate with the Salt master. It does so by
explicitly telling Salt to work with local grain data.

See <http://docs.saltstack.com/en/latest/ref/clients/#salt.client.Caller>
(especially the last example) and
<http://docs.saltstack.com/en/latest/ref/file_server/file_roots.html#local-file-server>.

Note that this issue affected passive examination of grains as well as
setting them.

Change-Id: I3d6ef6ed579f87d0f89eb1a43774a36bc6b0dace