Update store image file tests

*Replaced all references to self.file_data with self.test_file
*Made all calls to store_image_file more consistent
*Removed self.file_data variables from images fixture
*Removed cStringIO imports as needed
*Added error handling method for opening and reading from files

Change-Id: I70a7aa3e80f7a3e5285419ed8d18a69c4e668a50
Merge "Update store image file tests"