Replace hardcoded references with $mail_smarthost

People were apparently using smtp.pmtpa.wmnet, although that was our
alias to "wiki-mail", i.e. MediaWiki-generated email, which is why
these were initially missed. Switch them to $::mail_smarthost[0]
instead, which should properly point to our smarthosts. (Ideally, they
would be switched to use the sendmail interface, so the local MTA would
handle it with redundancy etc.)

This fixes:
- Gerrit
- Wikimania Scholarships
- snapshot/wikidump

Change-Id: Id21181e1791ea337fb3141f78be5aa3ae0fba9b0