I finally got ldap working for accounts both in ldap and /etc/passwd. Here is my /etc/dovecot.conf file, with all comments removed.

protocols = imaps pop3s
log_path = /var/log/dovecot.log
ssl_cert_file = /usr/share/ssl/certs/imapd.pem
ssl_key_file = /usr/share/ssl/certs/imapd.pem
login_dir = /var/run/dovecot/login
mmap_disable = yes
first_valid_uid = 200
protocol imap {
}
protocol pop3 {
}
protocol lda {
  postmaster_address = postmaster@example.com
}
auth default {
  mechanisms = plain
  passdb pam {
 }
passdb shadow {
}
passdb ldap {
  args = /etc/dovecot-ldap.conf
}
userdb passwd {
}
userdb ldap {
  args = /etc/dovecot-ldap.conf
}
   user = root
}
dict {
}
plugin {
}