Removal of SQLAlchemy dependency

[teiszler]

Originally we use a SQLAlchemy db to store all the relevant information: images, snapshots, pids of running instances, etc. I have started to pare back what we store in SQL by moving the pids into file under /var/nephele/pid, but we should be able to eventually remove this dependency by doing similar changes for images and snapshots. There are only a few pieces of important information stored for each, mainly the hash of the base image and a timestamp.

The images are already store in /var/nephele/images. If we create a subdirectory for each image which has the hash as the name, we should be able to quickly tell if an image exists simply by listing the contents of /var/nephele/images and performing a comparison. We could similarly store snapshots under /var/nephele/snapshots instead of user defined paths. The snapshots directory could have the same format with subdirectories for each image which contain the snapshots that were built from those images.