Replace ssmtp with msmtp

[rodrigoaguilera]

I did a brief investigation and seems like ssmtp is pretty abandoned these days and msmtp is the natural replacement.
I opened this issue to test the waters on how the replacement should happen in the image like

• The two packages living together while ssmtp is deprecated
• New env variables to configure msmtp

I'm willing to submit PRs to make this happen

[rodrigoaguilera]

Cool. Thanks!