This PR adds `ex_image_ref` to create_volume for the
OpenStack_2_NodeDriver. This allows the user to create
bootable volumes based on machine images. When the UUID
of an image is passed as ImageRef it will create a bootable
volume from which later an instance can be launched. This is
helpful because for older versions of OpenStack it is not
possible to create an instance with a block device mapping
where the newly created volume has to be put in a availability
zone that is not the default.

More info about bootable volumes at https://docs.openstack.org/api-ref/block-storage/v2/index.html?expanded=create-volume-detail#volumes-volumes

> To create a bootable volume, include the UUID of the image from which
> you want to create the volume in the imageRef attribute in the request
> body.

Currently it is not possible to use the create_node method without
specifying an image. This is because in the OpenStack_1_1_NodeDriver
create_node converts the server_params like:

```
server_params = self._create_args_to_params(None, **kwargs)
```

but in case when there is no image to boot from like when you are
booting from an already existing bootable volume (which could have been
created from an image earlier), then _create_args_to_params will try to
access node.extra in order to get the imageRef, and node is None:

```
        if 'image' in kwargs:
            server_params['imageRef'] = kwargs.get('image').id
        else:
            server_params['imageRef'] = node.extra.get('imageId')
```

Booting an instance from a previously existing bootable volume like this
would fail:

```
In [36]: conn.create_node(ex_availability_zone='R123', port='8487d948-0840-4205-8b31-7f705a19e7f4', name='r123apitestnode', ex_keyname
    ...: ='rick', size='e55a2688-ef74-44cf-b302-9a6f960c3d74', ex_blockdevicemappings=[{'boot_index': 0, 'uuid': 'be7ee330-b454-4414-8
    ...: e9f-c70c558dd3af', 'source_type': 'volume', 'destination_type': 'volume', 'delete_on_termination': False}])
```

with:
```
/usr/local/venv/hypernode-control/src/apache-libcloud/libcloud/compute/drivers/openstack.pyc in _create_args_to_params(self, node, **kwargs)
   1495             server_params['imageRef'] = kwargs.get('image').id
   1496         else:
-> 1497             server_params['imageRef'] = node.extra.get('imageId')
   1498
   1499         if 'size' in kwargs:

AttributeError: 'NoneType' object has no attribute 'extra'
```

This could be circumvented by specifying `image=''`:
```
In [39]: conn.create_node(ex_availability_zone='R123', port='8487d948-0840-4205-8b31-7f705a19e7f4', image='', name='r123apitestnode',
    ...: ex_keyname='rick', size='e55a2688-ef74-44cf-b302-9a6f960c3d74', ex_blockdevicemappings=[{'boot_index': 0, 'uuid': 'be7ee330-b
    ...: 454-4414-8e9f-c70c558dd3af', 'source_type': 'volume', 'destination_type': 'volume', 'delete_on_termination': False}])
```

This PR also changes the default imageRef to empty string '' instead of None to prevent the API from responding with an error like this when the .get would default to None so that `image=''` will now no longer have to be specified.
```
BaseHTTPError: 400 Bad Request Invalid input for field/attribute imageRef. Value: None. u'None' is not valid under any of the given schemas
```

…byte

Based on latest version of
https://raw.githubusercontent.com/apache/libcloud/trunk/libcloud/compute/constants/ec2_instance_types.py