Add "host" param for image panel

Corresponding to image api rework [1], also Zun UI need to add "host" param for image actions.
[1] https://blueprints.launchpad.net/zun/+spec/rework-image-api

So it requires retrieving host list, so this blueprint needs to implement its REST API and Angular service.

For now, only image pull action is implemented.
So this blueprint need to add "host" param into only image pull action.
Other actions for image will be implemented as having "host" param in the other blueprint.

Also, image list need to have "Host" column.