Merge branch 'stable-2.1' into devel-2.1

* stable-2.1:
  Bump version for 2.1.2.1 release
  Update NEWS for Ganeti 2.1.2.1
  KVM: only export instance tags if present

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>