Set default value for vg_options

Fixes a bug when vg_options is not specified:
  File "/home/vagrant/.ansible/tmp/ansible-tmp-1397283690.67-233519411783834/lvg", line 119, in main
    vgoptions = module.params.get('vg_options', '').split()
AttributeError: 'NoneType' object has no attribute 'split'