Do not check content-type of HTTP 204 messages

This fixes an error in commit 5b2277272dbb489eda5186e0853f7529d621c359.
Merge pull request #1303 from Ichimonji10/jenkins

Do not check content-type of HTTP 204 messages
Add API tests for config templates

Update the `ConfigTemplate` class definition: add mixins for creation, reading
and deletion. Thoroughly test the relevant API endpoints.