The method `Fernet.generate_key()` generates a binary string in Python 3:
```
>>> Fernet.generate_key()
b'qSMZlOK23pZUw_Uyy-ZRPUfPskMXKGCGmhG6AHCFiV8='
```

Unless properly written as a string to the Kolla `passwords.yml` file,
the Fernet key will end up in the final Barbican config like this:
```
[simple_crypto_plugin]
kek = b'qSMZlOK23pZUw_Uyy-ZRPUfPskMXKGCGmhG6AHCFiV8='
```

Due to the fact that the key is incorrectly written to the barbican
config file (it should be written as a string), every barbican secret
store fails with:

```
barbican.api.controllers   File "/var/lib/kolla/venv/lib/python3.6/site-packages/barbican/plugin/store_crypto.py", line 83, in store_secret
barbican.api.controllers     encrypting_plugin, context.project_model)
barbican.api.controllers   File "/var/lib/kolla/venv/lib/python3.6/site-packages/barbican/plugin/store_crypto.py", line 290, in _find_or_create_kek_objects
barbican.api.controllers     kek_meta_dto = plugin_inst.bind_kek_metadata(kek_meta_dto)
barbican.api.controllers   File "/var/lib/kolla/venv/lib/python3.6/site-packages/barbican/plugin/crypto/simple_crypto.py", line 104, in bind_kek_metadata
barbican.api.controllers     encryptor = fernet.Fernet(self.master_kek)
barbican.api.controllers   File "/var/lib/kolla/venv/lib/python3.6/site-packages/cryptography/fernet.py", line 38, in __init__
barbican.api.controllers     "Fernet key must be 32 url-safe base64-encoded bytes."
barbican.api.controllers ValueError: Fernet key must be 32 url-safe base64-encoded bytes.
```

This commit fixes the issue described above by properly writing
the Fernet key as a string to the Kolla `passwords.yml` file.

Closes-Bug: #1848191
Change-Id: I27fc0159c889bc2e1576fdd69b7d02a320b620f8