The disk field 'physical_id' has to be kept up to date whenever a disk
object is sent to a node via RPC. This is done with the SetDiskID method
manually, which is a source of bugs.

This patch replaces the use of 'physical_id' with a new field names
'dynamic_params'. The RPC code is adapted to update this field whenever
a disk object is sent to a node. Furthermore, this field is only ever
set on copies of disk objects which don't get written to the
configuration file. On the node side, the use of 'physical_id' is
removed and the new dynamic parameters are used now for the same
purpose.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Jose A. Lopes <jabolopes@google.com>