Use proper headers to auth with Vault

Reading examples at
https://learn.hashicorp.com/vault/getting-started/apis show needing to
use `X-Vault-Token` header, instead of `Authorization`. Without this
header, the vault server would return a 400 status with an error message
of "missing client token". With this change AWX is now able to interface
with the Hashicorp backend.

awx/main/credential_plugins/hashivault.py

@@ -102,7 +102,7 @@ def kv_backend(**kwargs):
         request_kwargs['verify'] = create_temporary_fifo(cacert.encode())
 
     sess = requests.Session()
-    sess.headers['Authorization'] = 'Bearer {}'.format(token)
+    sess.headers['X-Vault-Token'] = token
 
     if api_version == 'v2':
         if kwargs.get('secret_version'):
@@ -157,7 +157,7 @@ def ssh_backend(**kwargs):
         request_kwargs['json']['valid_principals'] = kwargs['valid_principals']
 
     sess = requests.Session()
-    sess.headers['Authorization'] = 'Bearer {}'.format(token)
+    sess.headers['X-Vault-Token'] = token
     # https://www.vaultproject.io/api/secret/ssh/index.html#sign-ssh-key
     request_url = '/'.join([url, secret_path, 'sign', role]).rstrip('/')
     resp = sess.post(request_url, **request_kwargs)