issue #587: mitogen_doas should not become_exe for doas_path

Looks like this has always been wrong - when used as a connection
method, PlayContext.become_method/become_exe may hold totally unrelated
data.

ansible_mitogen/connection.py

@@ -356,7 +356,7 @@ def _connect_mitogen_doas(spec):
             'username': spec.remote_user(),
             'password': spec.password(),
             'python_path': spec.python_path(),
-            'doas_path': spec.become_exe(),
+            'doas_path': spec.ansible_doas_exe(),
             'connect_timeout': spec.timeout(),
             'remote_name': get_remote_name(spec),
         }

ansible_mitogen/transport_config.py

@@ -294,6 +294,12 @@ class Spec(with_metaclass(abc.ABCMeta, object)):
         Connection-specific arguments.
         """
 
+    @abc.abstractmethod
+    def ansible_doas_exe(self):
+        """
+        Value of "ansible_doas_exe" variable.
+        """
+
 
 class PlayContextSpec(Spec):
     """
@@ -425,6 +431,12 @@ class PlayContextSpec(Spec):
     def extra_args(self):
         return self._connection.get_extra_args()
 
+    def ansible_doas_exe(self):
+        return (
+            self._connection.get_task_var('ansible_doas_exe') or
+            os.environ.get('ANSIBLE_DOAS_EXE')
+        )
+
 
 class MitogenViaSpec(Spec):
     """
@@ -635,3 +647,9 @@ class MitogenViaSpec(Spec):
 
     def extra_args(self):
         return []  # TODO
+
+    def ansible_doas_exe(self):
+        return (
+            self._host_vars.get('ansible_doas_exe') or
+            os.environ.get('ANSIBLE_DOAS_EXE')
+        )