Encode pubkey in validators

Follow up change for py2->py3 migration.
The pubkey must be encoded in order to avoid 'data must be
bytes-like' error. The key is a type of str which in py2 meant
bytes, however, in py3 it's just string. Therefore the pubkey
must be encoded before it's passed to load_ssh_public_key which
requires bytes-like data.

Change-Id: I943438a59923bd9c4e9dd38058020e49fd2f3d24

refstack/api/validators.py

@@ -126,8 +126,14 @@ class TestResultValidator(BaseValidator):
                 raise api_exc.ValidationError('Malformed signature', e)
 
             try:
+                pubkey = request.headers.get('X-Public-Key', '')
+                try:
+                    pubkey.encode('utf-8')
+                except AttributeError:
+                    # it's already in bytes
+                    pass
                 key = load_ssh_public_key(
-                    request.headers.get('X-Public-Key', ''),
+                    pubkey,
                     backend=backends.default_backend()
                 )
             except (binascii.Error, ValueError) as e: