send email to admins of missing slaves

[warner]

If a buildslave is connected but then goes away for more than a day, send email to the admin of the missing buildslave.

Often we have buildslaves go away because the owner of the machine in question reboot their machine and forgot to restart the slave (and failed to arrange for it to start automatically).

This depends upon #63.

I'm thinking that the BuildSlave class defined in #63 should have two additional arguments:

s = BuildSlave("slavename", "slavepassword", admin_email="slave-admin@example.org",
               notify_after=24*60*60)

Where notify_after= is the number of seconds of continuous outage that must pass before an email is sent. The email will be sent once per outage block (i.e. if the slave stays away forever, they'll only get one email, but if it is online on mondays and not for the rest of the week, they'll get a message every tuesday night). notify_after=None disables the feature.

At some point in the future we should add a buildmaster_admin_email to the config file, and they can get notified about these problems too.

[warner]

Done, in [28ace0d03957289e1b6e791c80ddd37aef0d6b3c]. This will use an existing MailNotifier? if it can find one, which provides a way to get a relayhost and fromaddr set. Otherwise it uses localhost and a fromaddr of "buildbot" (which should get qualified by the MTA listening on localhost.