Introduce missing fields on StorageVolume and Image (#623)

The integration tests are showing many warnings similar to `UserWarning:
Attempted to set unknown attribute "type" on instance of "Image"`, due
to PyLXD not recognizing some newish fields from a few API responses.
This introduces such fields to PyLXD.

pylxd/models/image.py

@@ -49,6 +49,9 @@ class Image(model.Model):
     size = model.Attribute(readonly=True)
     uploaded_at = model.Attribute(readonly=True)
     update_source = model.Attribute(readonly=True)
+    type = model.Attribute(readonly=True)
+    project = model.Attribute(readonly=True)
+    profiles = model.Attribute(readonly=True)
 
     @property
     def api(self):

pylxd/models/storage_pool.py

@@ -300,6 +300,10 @@ class StorageVolume(model.Model):
     config = model.Attribute()
     used_by = model.Attribute(readonly=True)
     location = model.Attribute(readonly=True)
+    # Date strings follow the ISO 8601 pattern
+    created_at = model.Attribute(readonly=True)
+    pool = model.Attribute(readonly=True)
+    project = model.Attribute(readonly=True)
 
     snapshots = model.Manager()