#1071: fix BQ system tests under Py3k

)](GoogleCloudPlatform/python-docs-samples#1074)

The use of base64 is essentially an implementation detail of the Cloud KMS REST
API: it is required only so that arbitrary binary data can be included in a JSON
string, which only allows Unicode characters. Therefore, the "encrypt" sample
function should decode the base64-encoded ciphertext before writing the
file. Similarly, "decrypt" should not assume that an input file is
base64-encoded, but should perform the base64-encoding itself before sending the
encrypted data to KMS.

This aligns with how the "gcloud kms encrypt" and "gcloud kms decrypt" commands
behave. See https://stackoverflow.com/q/45699472 for an example of user
confusion caused by the mismatch.