Add a monitor type which can check for expected remote instances

[jamesoff]

Currently, if a remote instance stops running (and everything else about the host it's on is ok) then the only clue that something is wrong is that its monitors will be reported as OLD in the HTML logger output. If the main monitor instance is restarted, then the remote monitors will disappear and unless they are explicitly looked for, their absence could be missed.

Add a monitor type, or some special configuration for the main SimpleMonitor class, which describes the remote monitor instances we expect to see reporting to us, and fails when they're not.

As part of this, remote instances could pass metadata about themselves (e.g. SimpleMonitor version, host information, Python version) which can be inspected to find out-of-date installation.s