Explicitly ask for the default iallocator in commands

Now "gnt-instance recreate-disks" uses the default iallocator when "." is
specified as the iallocator. For uniformity, the same behavior applies to
these commands:
  gnt-node evacuate
  gnt-instance migrate
  gnt-instance add
"." is used instead of "default" becuse the latter could be a valid name for
an iallocator.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>