Support configure multi-backend in cinder-volume

Current block-storage cookbook does not support configure multiple backends in cinder.conf. The blueprint is going to add this configuration followed by the instructions described in specification.

1. Enable 'enabled_backends' list option in template cinder.conf.erb at the end of [DEFAULT] section
2. Add a hash attribute to cookbook-openstack-block-storage, which will be used by template to generate the final configuration

This hash attribute will look like this:
node['openstack']['block-storage']['multi-backend'] = {
'lvmdriver-1':
    { 'volume_group': 'stack-volumes-1',
      'volume_driver': 'cinder.volume.drivers.lvm.LVMISCSIDriver',
      'volume_backend_name': 'LVM_iSCSI'}
}
3. Modify cinder volume scheduler as default 'cinder.scheduler.filter_scheduler.FilterScheduler'
4. (Optional) Add a recipe to create volume type into database