spamassassin: add bayes path

We run spamd with --nouser-config and SA's config defaults to storing
the bayes database to ~/.spamassassin/bayes, which results in spamd
trying to access nonexistent/.spamassassin/bayes.mutex. Set the
bayes_path globally, to /var/lib/spamassassin, to prevent this from
happening.

Change-Id: I25876e775a07732f4db947cb333182d5d8f47f81