Record the move being performed in a Placement
Record the move being performed in a Placement

This will allow a more descriptive output later in the solution list, as
opposed to trying to reconstruct the move from the node indices.

The patch also documents the Placement members.

andir/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,apyrgio/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti/ganeti,leshchevds/ganeti,grnet/snf-ganeti,andir/ganeti,andir/ganeti,ganeti/ganeti,dimara/ganeti,dimara/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,yiannist/ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti