Medium: pengine: Correctly perform partial migrations when node's uname is not equal to the id.

The migrate_source and migrate_target metadata attributes for
the migration operations represents the node's uname, not the
corosync id.  The node's id was being used when detecting
a partial migration, causing partial migrations to fail
when uname!=id.
Merge pull request #411 from davidvossel/master

Medium: pengine: Correctly perform partial migrations when node's uname is not equal to the id.