salt: Vault module problem in 2018.3.0

Description of Issue/Question

vault.read_secret fails when 2017 minion runs against a 2018.3 salt-master.

Minion:

[ERROR   ] Failed to get token from master! Response is not a dict: Exception occurred in runner vault.generate_token: Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/salt/client/mixins.py", line 387, in _low
        data['return'] = self.functions[fun](*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/salt/runners/vault.py", line 46, in generate_token
        _validate_signature(minion_id, signature, impersonated_by_master)
      File "/usr/lib/python2.7/site-packages/salt/runners/vault.py", line 129, in _validate_signature
        'Could not validate token request from {0}'.format(minion_id)
    AuthenticationError: Could not validate token request from **MINION-ID**

And master logs sometimes show

2018-04-17 16:17:45,168 [salt.loaded.ext.utils.vault:45  ][ERROR   ][9979] Failed to get token from master! An error was returned: list indices must be integers, not unicode
2018-04-17 16:17:45,169 [salt.loaded.ext.module.vault:47  ][ERROR   ][9979] Failed to read secret! CommandExecutionError: {u'error': u'list indices must be integers, not unicode'}

In trying to troubleshoot the _validate_signature function, I added debug lines to print out the minion signature and got:

2018-04-17 16:26:06,114 [salt.loaded.int.runners.vault:126 ][ERROR   ][9990] Validating signature for **MINION-ID**
2018-04-17 16:26:06,115 [salt.loaded.int.runners.vault:128 ][ERROR   ][9990] Got signature q����r�"w��|��A����f�x�RO-����kc6�   ݨS6]c�֯�q�ӑ�70�@%5�"�\�=gE̓�/�fEp�?JI*����nݶQ";�߭��dU�9�#��C�[$� ���7�~(j�S�4#�4�M|L���?�u�.uT�T�~Y�~�
�4�ňʅhkH���E�          ,�j�`=   � b�!d$�t�O�.)sΩ,��H

At this stage I’m guessing there is an issue with the strings being passed, and the recent unicode updates from 2018.

<del>Additionally, to get this far, I had to correct some typos in the vault runner code for which I’ll submit a PR. </del> Corrected recently in https://github.com/saltstack/salt/commit/01a5b88e7b71b109b4708f74e1796e88eea67bf7