Lowercase all image names for Docker driver

Docker repository names must be lowercase. When
an image is requested or a snapshot is created,
the requested name should be lowercased such that
it may be given a valid name in the Docker registry.

Adds tests for snapshots and get_image_name.

Change-Id: I97b281d30454e33ba32e428901bb5da58400308f
Closes-bug: 1284761
Merge "Lowercase all image names for Docker driver"