Support default network modes

In the case of EC2 launch type instances with no custom
network modes specified, the response of describe-task-definition
does not include a networkMode key. In those cases, we can
skip the AWSVPC block entirely. This adds tolerance for the
absence of that key.

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add support for Django 3.2, 4.2, 5.0 [#33](https://github.com/azavea/django-ecsmanage/pull/33)
 
 ### Changed
+- Support default network modes [#33](https://github.com/azavea/django-ecsmanage/pull/33)
 - Upgrade CI to use newer GitHub Actions for `checkout` and `setup-python` [#33](https://github.com/azavea/django-ecsmanage/pull/33)
 
 ### Removed

ecsmanage/management/commands/ecsmanage.py

@@ -185,7 +185,7 @@ def run_task(self, config, task_def_arn, security_group_id, subnet_id, cmd):
 
         # Only the awsvpc network mode supports the networkConfiguration
         # input value.
-        if task_def["networkMode"] == "awsvpc":
+        if task_def.get("networkMode") == "awsvpc":
             kwargs["networkConfiguration"] = {
                 "awsvpcConfiguration": {
                     "subnets": [subnet_id],