Instance pinning is introduced in ganeti locations design document.
It adds new instance tag of form htools:desiredlocation:x where x is
a location tag of a desired primary node. This implemented by adding
second component to the instance locationScore.
The metric is extended with the following component:

* Sum of dissatisfied desired locations number among all cluster instances.
  An instance desired location is dissatisfied when the instance is assigned
  a desired-location tag x where the node is not tagged with the location tag
  x.

Signed-off-by: Oleg Ponomarev <onponomarev@gmail.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>