Dkim Update

[SaraSmiseth]

Description

This PR adds the ability to specify the DKIM selector. If not provided, it falls back to the until now default mail. Also this fixes hardware#408 aswell.

In Addition, the OPENDKIM_KEY_LENGTH environment variable has been changed to DKIM_KEY_LENGTH to match the new DKIM_SELECTOR. For backwards compatibility it falls back to OPENDKIM_KEY_LENGTH if not defined (which again defaults to 1024).

With this PR the location of the DKIM keys has changed (with automatic migration). They are in the same folder but are now named selector.private.key and selector.public.key. So its not a breaking change, but if someone is automatically using the keys somewhere else, this might need adjustment to the new key names.

Fixes # (issue)

* Until now the DKIM selector was always mail and could not be changed.

* Now the DKIM selector can be changed and falls back to mail for backwards compatibility

Type of change

Please delete options that are not relevant.

* [x]  Bug fix (non-breaking change which fixes an issue)

* [x]  New feature (non-breaking change which adds functionality)

Status

* [x]  Ready

* [ ]  In development

* [ ]  Hold

Todo List

How has this been tested ?

This has been tested with the (updated) internal test suite.

See hardware#417