Make DNS understaind easier for users

[n-tdi]

This simple change makes it easier for new users to add the DNS records they need for there Modoboa installation.

Description of the issue/feature this PR addresses: The original issue was that people had a hard time understanding what the Name for the MX record should be. The @ displays that it is the root of the domain; which people are able to comprehend better.

Current behavior before PR: Does not display a @ symbol in front of the MX record.

Desired behavior after PR is merged: Displays an @ symbol before the IN MX mail.example.com.

[n-tdi]

Hey @tonioo , Can this get merged?

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (63d92b7) 50.07% compared to head (cac6c1e) 50.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #479 +/- ## ======================================= Coverage 50.07% 50.07% ======================================= Files 10 10 Lines 681 681 ======================================= Hits 341 341 Misses 340 340 ``` | [Impacted Files](https://codecov.io/gh/modoboa/modoboa-installer/pull/479?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa) | Coverage Δ | | |---|---|---| | [run.py](https://codecov.io/gh/modoboa/modoboa-installer/pull/479?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa#diff-cnVuLnB5) | `55.63% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa)

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