Fix inconsistent "image" value on _get_image()

The type of "image" value is different on showing details of a server,
such as:
1. It's a dict while creating a server from a image
2. It's a empty string while creating a server from a volume

We should modify the type to dict, otherwise someone who call
the api will be confused about the value or can't parse the value
in the same way.

The patch just modifies the v3 api.

Change-Id: I0d7ce59f64adec0933353af58dc5b5170390674b
Closes-Bug: #1262039
Merge "Fix inconsistent "image" value on _get_image()"