Instance customization script can now be uploaded as file

Added the ability for a user to upload a customization script file from
their local machine instead of copy/pasting it into a text field if they
prefer.

Tests are included to cover this new functionality.

Change-Id: Icc0e750346822a26ea853d4cc3d790d9d9f289d5
Closes-Bug: 1298483
Merge "Instance customization script can now be uploaded as file"