Make tests' encrypt_secret.py work with python3

This was not compatible with python3 due to encoding issues of the input
and the out. Ensure we pass the input plaintext as bytes to the
encryption routine and use the base64 module to convert the output to
base64.

Change-Id: Ie8b3a8e5c93544e448016829c1071240b68e8957
1 file changed