RBD store uses common utils for reading file chunks

Prevents cases where the image_size passed to the RBD store is larger
than the actual image_file which attempts to write 0 bytes of data
causing image.write to hang indefinitely.

Fixes LP Bug #1146830

Cherry-picked from havana (101f000d3ca25761d75f626fe4379143ef5a6a70)

Change-Id: I56216437a212c0c802d2ed349d88b3bd0d3bb4d7
Merge "RBD store uses common utils for reading file chunks" into stable/grizzly