Increase UT Coverage for magnumclient and add coverage job to CI

Magnumclient has low Test coverage(around 67%). As many features are added/updated in magnum, magnumclient will also have significant impact. Their is higher chances nothing breaks with these changes, it is required to increase coverage first and then enforce adding tests for new changes.
The coverage can be targeted to be increased upto 90% and add a non-voting CI job. Once we get it then tests can be further updated and make it voting by checking missing tests in each patch set.