BUG: Registration interface failed multi-modal

[hjmjohnson]

Registration interface was failing to support multimodal input when they are given.

ex)
nipype:
  antsWF.inputs.fixed_image=['fixed_t1.nii.gz',
                             'fixed_t2.nii.gz']
  antsWF.inputs.moving_image=['moving_t1.nii.gz',
                              'moving_t2.nii.gz']

corresponding correct commandline:
  --metric MI[\
      fixed_t1.nii.gz,\
      moving_t1.nii.gz,\ ...
  --metric MI[\
    fixed_t2.nii.gz,\
    moving_t2.nii.gz,\ ...

[oesteban]

Fixed in #1176