Handle correctly offline nodes in cluster scoring
Handle correctly offline nodes in cluster scoring

This patch changes two things with regard to offline nodes:
  - first, it only calculates the various coefficients across online
	nodes
  - second, it adds a new score denoting the percentage of instances
	which live on such nodes

The first change allows correct score computation in presence of offline
nodes (whose properties we don't need to take into account), while the
second change actively evacuates offline nodes.
andir/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,apyrgio/snf-ganeti,yiannist/ganeti,grnet/snf-ganeti,onponomarev/ganeti,ganeti/ganeti,mbakke/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,dimara/ganeti,bitemyapp/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,dimara/ganeti,leshchevds/ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti/ganeti,mbakke/ganeti,apyrgio/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,andir/ganeti