Fix batcher for 2.0-style disks and nics

This patch fixes the gnt-instance batch-create command, and in doing so
also slightly changes two other functions:
  - we change utils.ParseUnit so that it accepts integer values also
    (both ParseUnit(5) and ParseUnit("5") return the same value)
  - a bridge 'None' in LUCreateInstance will be converted to the default
    bridge; currently only missing bridges will be accepted to mean the
    default one

The main changes to batcher were the change to variable number of disks
and NICs.

The patch also adds a batcher-instances.json example file copied from
the 1.2 branch and properly modified.

Reviewed-by: imsnah, killerfoxi