docker build is failure in lint step

[Mizuki Urushida (zuiurs)]

what's happend

In processing Dockerfile, the step of flake8 check was failure.

$ docker build --tag ultimaker/k8s-mongo-operator:local .
<output omitted>

Step 15/21 : RUN flake8 . --count
 ---> Running in a6c3775ae6e1
./tests/services/TestMongoService.py:119:31: Q003 Change outer quotes to avoid escaping inner quotes
./tests/services/TestMongoService.py:190:13: Q003 Change outer quotes to avoid escaping inner quotes
./tests/services/TestMongoService.py:206:13: Q003 Change outer quotes to avoid escaping inner quotes
./tests/services/TestMongoService.py:211:26: Q003 Change outer quotes to avoid escaping inner quotes
4
The command '/bin/sh -c flake8 . --count' returned a non-zero code: 1

what this PR does

replace double quotes to single quotes in some line where includes back slash for escaping innner quotes.