Allow disk object to set their own physical ID

Currently, the way to customize a DRBD disk from (node name 1, node name
2, port) to (ip1, port, ip2, port) is to use the ConfigWriter method
SetDiskID. However, since this needs a ConfigWriter object, it can be
run only on the master, and therefore disk object can't be passed to
more than one node unchanged. This, coupled with the rpc layer
limitation that all nodes in a multi-node call receive the same
arguments, prevent any kind of multi-node operation that has disks as an
argument.

This patch takes the SetDiskID method from ConfigWriter and ports it to
the disk object itself, and instead of the full node configuration it
uses a simple {node_name: replication_ip} mapping for all the nodes
involved in the disk tree (currently we only pass primary and secondary
node since we don't support nested drbd devices).

This allows us to send disks to both the primary and secondary nodes at
once and perform synchronized drbd activation on primary/secondary
nodes.

Note that while for the 1.2 branch this will not change old methods, it
is worth to investigate and possible replace all such calls on the
master to the nodes themselves for the 2.0 branch.

Reviewed-by: ultrotter