Support variable interpolation for volumes and networks sections.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

tests/unit/config/interpolation_test.py

@@ -0,0 +1,69 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import os
+
+import mock
+import pytest
+
+from compose.config.interpolation import interpolate_environment_variables
+
+
+@pytest.yield_fixture
+def mock_env():
+    with mock.patch.dict(os.environ):
+        os.environ['USER'] = 'jenny'
+        os.environ['FOO'] = 'bar'
+        yield
+
+
+def test_interpolate_environment_variables_in_services(mock_env):
+    services = {
+        'servivea': {
+            'image': 'example:${USER}',
+            'volumes': ['$FOO:/target'],
+            'logging': {
+                'driver': '${FOO}',
+                'options': {
+                    'user': '$USER',
+                }
+            }
+        }
+    }
+    expected = {
+        'servivea': {
+            'image': 'example:jenny',
+            'volumes': ['bar:/target'],
+            'logging': {
+                'driver': 'bar',
+                'options': {
+                    'user': 'jenny',
+                }
+            }
+        }
+    }
+    assert interpolate_environment_variables(services, 'service') == expected
+
+
+def test_interpolate_environment_variables_in_volumes(mock_env):
+    volumes = {
+        'data': {
+            'driver': '$FOO',
+            'driver_opts': {
+                'max': 2,
+                'user': '${USER}'
+            }
+        },
+        'other': None,
+    }
+    expected = {
+        'data': {
+            'driver': 'bar',
+            'driver_opts': {
+                'max': 2,
+                'user': 'jenny'
+            }
+        },
+        'other': {},
+    }
+    assert interpolate_environment_variables(volumes, 'volume') == expected