Port image-size extension to work in v2.1/v3 framework

Ports v2 image-size extension and adapts it to the v2.1/v3 API
framework. API behaviour is identical with the exception
that there is no support for XML. Also

- unittest code modified to share testing with both v2/v2.1
- Adds expected error decorators for API methods

Partially implements blueprint v2-on-v3-api

Change-Id: I44f3d35b3d888e2f4429ca0788d74b9b0cce9494
Merge "Port image-size extension to work in v2.1/v3 framework"