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
diff --git a/tests/encrypt_secret.py b/tests/encrypt_secret.py
index 0b0cf19..4bc514a 100644
--- a/tests/encrypt_secret.py
+++ b/tests/encrypt_secret.py
@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import base64
 import sys
 import os
 
@@ -27,8 +28,10 @@
         private_key, public_key = \
             encryption.deserialize_rsa_keypair(f.read())
 
-    ciphertext = encryption.encrypt_pkcs1_oaep(sys.argv[1], public_key)
-    print(ciphertext.encode('base64'))
+    plaintext = sys.argv[1].encode('utf-8')
+
+    ciphertext = encryption.encrypt_pkcs1_oaep(plaintext, public_key)
+    print(base64.b64encode(ciphertext).decode('utf-8'))
 
 
 if __name__ == '__main__':