Add collectd email notification plugin support

* New variable: `swift_mail_recipients` which is a list of email addresses.

* New variable: `swift_collectd_email_notifications` (default false)
  that must be true to enable the email plugin.

* New variable: `swift_mail_from_address` (default
  monitoring@hostname) that will be used as the from address on emails
  sent from monitoring / cron systems (currently only used by monitoring).