Add support for automatically installing `bsd-mailx` if mail is to be sent

[colans]

Is there any reason not to install bsd-mailx if unattended_mail has been set to TRUE?

This is the package I've always used for this, when configuring this functionality manually. I'd like to start using this role, but that package installation is missing. It shouldn't be necessary to install that and then run this role.

We could introduce variables like:

• install_mailer (boolean, defaults to False)
• mailer_package (defaults to bsd-mailx)

I could put together a PR if there are no objections.

[colans]

Actually, I think we can get by with msmtp-mta along with the bas-mailx package, which would get installed by another role. Sorry for the trouble.

[jnv]

Thanks. If I remember correctly the unattended-upgrades package recommends a few MTAs, so I'd prefer to leave the choice to user. It could be documented for this role, but I don't think we need to replace all the existing documentation for unattended-upgrades… 🙂