Add support for preseeding minion id
Also set `$_SALT_ETC_DIR/minion_d`

This is technically not necessary since `id:` overrides it, but I think
it's more transparent.
Not preseeding `99-minion-id.conf`

It's not necessary. `$_SALT_ETC_DIR/minion_id` is enough.
Make sure $_SALT_ETC_DIR is created

Defensive programming...
Merge pull request #348 from JensRantil/preseed-minion-id

Preseed minion id