[images] Fix create_default_topology condition check #421

[keviiin38]

Fix if condition checking if the default topology should be created.

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #421

Description of Changes

Currently the if condition returns True even if the variable USE_OPENWISP_TOPOLOGY is set to False.
This is due to the fact that the value returned is "False", which evaluates to True in Python.

Screenshot

n/a