Make eventlet hub choice configurable.

Some platforms do not support the 'poll' hub.

Change-Id: Ibd6abe499c7f6fdd84f5626b7e8fcb756e45ed57
Adding store location proxy to domain

Adding a proxy layer to image to take care of it's locations for the
store:
1. Image location correctness checking.
2. Remove image data from the store when a location is removed from an
image.

Implement bp: multiple-image-locations

Change-Id: Ifa8b19050b48d1099aa37e77d144d10e6587b99c
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Make eventlet hub choice configurable."
Merge "Adding store location proxy to domain"