Add test cases for list deployments. Fixes T719

Summary:
 - Mock test for ListDeployment execution method.
 - Mock test for ListDeployment run method.
 - Mock test when a null parameter is passed to the ListDeployment execution method.

Test Plan:
Run
 - `./gradlew test`
 - See test pass

Reviewers: dkobia, PapaLee

Subscribers: shadowhand, rjmackay

Maniphest Tasks: T719

Differential Revision: https://phabricator.ushahidi.com/D486