In #15870 we got good feedback that it'd be more useful to have the
various filename-accepting arguments on this provisioner instead accept
strings that represent the contents of such files, so that they can be
generated from elsewhere in the Terraform config.
This change does not achieve that, but it does make room for doing this
later by renaming "minion_config" to "minion_config_file" so that we
can later add a "minion_config" option alongside that takes the file
content, and deprecate "minion_config_file".
Ideally we'd just implement the requested change immediately, but
unfortunately the release schedule doesn't have time for this so this is
a pragmatic change to allow us to make the full requested change at a
later date without backward incompatibilities.
This change is safe because the salt-masterless provisioner has not yet
been included in a release at the time of this commit.
6a4498ba76