Add a rpc call for drbd network reconfiguration

Add a rpc call for drbd network reconfiguration

This patch adds a rpc call and backend.py implementation over the
low-level bdev.DRBD8.ReAttachNet() call.

The rpc call and reconfiguration is done in such a way that it needs to
be called to two nodes in parallel, and will wait (with one minute
timeout) until the two nodes are reconnected with the new configuration.
Note that this means that if the rpc calls cannot be launched in
parallel to the two nodes, this call will fail.

The patch also adds an extra attribute to the bdev.DRBD8Status class,
and changes the blockdev_close rpc to also pass the instance name to
backend.CloseBlockDevices(), in order to cleanup the symlinks (which are
create in the DrbdReconfigNet function).

Reviewed-by: imsnah