This function reads a zip file and populates the keystores with the keys
in the zip file. Root keys are left encrypted, and non-root keys are
decrypted before being added to the keystore.
The unit test first exports a repo's keys to a zip file, then imports it
into another repo. It checks that all the correct keys exist in the new
repo after the import operation.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>