OTP’s default SSL/TLS settings are rather restricitive
and in particular do not use system CA certs.
In our case using system CA certs is virtually always desired
and the lack of it leads to non-obvious errors. Manually configuring
system CA certs from in-database config also isn’t straightforward.
Furthermore, gen_smtp uses a different set of connection options
for direct SSL/TLS and a later TLS upgrade providing additional
confusion and complexity in how to configure this.
Thus provide some suitable defaults for sending SMTP emails.
Everything can still be overriden by admins if necessary.
Note: defaults are not appended when validating the config
in hopes of improving the error message (as the required relay key
is already accessed to generate defaults for optional fields)
Fixes: #660
