Refactor: pacemaker-attrd: track node CIB ID rather than cluster ID

Previously, attribute_value_t had a nodeid member to track the cluster
ID of the node that the value is for. However the only reason we need
it is to be able to get the node's XML ID in the CIB, for writing out
the value. Rename it to node_xml_id and track the XML ID directly.

In practice, there is no real change, since the CIB XML ID of Corosync
nodes is simply their cluster ID as a string. This allows us to keep
the same XML attribute and value in peer messages for backward
compatibility.

If we ever support node XML IDs that are *not* the string equivalent of
their cluster IDs, rolling upgrades will be possible only from versions
with this commit and later.