Add API tests for config templates

Update the `ConfigTemplate` class definition: add mixins for creation, reading
and deletion. Thoroughly test the relevant API endpoints.
Merge pull request #1301 from Ichimonji10/test-config-template

Add API tests for config templates
Test API for template kinds

Test whether it is possible to issue an HTTP GET request to
`api/v2/template_kinds`.
Merge pull request #1306 from Ichimonji10/test-templatekind

Test API for template kinds