and destroy a grain previously set to None, with destructive=True:
The f46722aaeb commit, from Bruno Binet <bruno.binet@gmail.com> was
effectively reverted during my work on nested and complex values support.
This commit get it back, plus the ability to destroy a existing grain that
was previously set to None, plus tests for these.
This way, there is no /tmp/grains created on test execution. And the issue
with test_absent_delete is always triggered.
Maybe we should use mocked files, but I'm not sure how to do it.
There are many jenkins errors on linode CentOS7. The function `saltutil.sync_grains`
does not seem to be mocked correctly on this platform.
Based on 1f5836d0bb
