Some improvements to gnt-node repair-storage

Currently the repair storage has two issues:

- down instances are aborting the operation, even though they should be
  ignored (it's not technically possible to know their disk status
  unless we would activate their disks)
- if the VG is so broken that disks cannot be activated via gnt-instance
  activate-disks or gnt-instance startup, it's not possible to repair
  the VG at all

The patch makes the opcode skip down instances and also introduces an
``--ignore-consistency`` flag for forcing the execution of the LU.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>