Improve the coverage of unit tests in mistral and its pythonclient

There are few unit tests in Mistral and there are lot of code are not covered in unit tests. Same case with python-mistralclient.

This BP is to improve the coverage of unit tests in both repo. All unit tests patches will be tracked under this BP.

renat: I am not sure I agree with "few unit tests". Could you be more specific what's covered and what's not from your perspective? Maybe some coverage tool report?

Lucky: when i have seen the unit test cases then i found almost positive cases covers but some few negative cases not covered yet . so i will write few miss cases so that if any issue occurred then we can handle at the time of UT . because if cover negative cases then chances of leakage of bugs will be reduce .