device-plugin: dynamic devices quantity

[Tal-or]

With the current implementation we the device plugin makes sure to populate more shared cpu devices in case that all devices are allocated.
The problem with this approach is that it might lead to potential unbound growth of devices.
As for now there's a limit for thousand devices which is sufficient but we want to make the device-plugin smarter and shrink the pool of devices when devices gets released.

The idea about how to do that described here: #7 (comment)