Create reproducible server image used for generic driver

Currently the image used for testing/gating Manila with the generic driver is placed here: https://www.dropbox.com/s/vi5oeh10q1qkckh/ubuntu_1204_nfs_cifs.qcow2 . The image is maintained by a single person and it's not obvious how this image was created.
Create a new repository which contains the code to reproducible build an image suitable for testing/gating Manila.