Make ping more reliable.

[marcosnils]

Ping now creates a session and an instance once a minute.

@xetorthio ?

[xetorthio]

This looks very promising. I think we should still check for available ram and hdd space before trying to create a session and instance.
The reasoning is that if you try to create a session or instance and it fails because of ram or hdd, probably is already too late, and we want to catch it before that. So we would put a good enough threshold, so we get notified and stop scheduling session in the instance.

[marcosnils]

makes sense. I'll update and resend.

[marcosnils]

I think we should still check for available ram and hdd space before trying to create a session and instance.

We can't really check HDD space in /ping as it's not exposed in /proc.