Add Pooling for multiple volume repositories

[Joacchim]

Currently, the driver only supports having a fail-over list or servers.

We need to be able to define pools that would allow using different volume repositories at the same time from the same driver. Those pools may need to be saved locally for reload purposes. For instance, here is how the /sys filesystem could look:

/sys/class/dewb/
               /debug
               /pools/
                     /add
                     /remove
                     /pool1/
                           /add_mirrors
                           /remove_mirrors
                           /create
                           /destroy
                     /pool2/
                           /add_mirrors
                           /remove_mirrors
                           /create
                           /destroy
...

[scality]

all parameters (timeouts, mirros, etc) shall be pool specific