Mailtest improvements: Don't rely explicitly on osparc.io SMTP server

[mrnicegyu11]

Currently all mail-tests (that mainly test that a mail can be recieved at this point) for sending mails rely in a hardcoded manner on the osparc.io SMTP server.

Ideally all deployments should

• Have their own SMTP server or one (like mail.speag.com) associated with them. This is easy for aws/tf deployments I guess, but requires coordination for on-prem
• test that their associated receiving (IMAP) and sending (SMTP) servers work by sending and receiving a mail there (currently only the receiving part works very well, although arguably the sending part is more important for the osparc-simcore platform)

FYI @YuryHrytsuk