Tenants API User model

    1. Added user as a new tenants-api responses model.
    2. Added user creation as a tenants-api client functionality
    3. Tested (mock) user creation on a tenants-api client
    4. Added  user role creation as a tenants-api client functionality
    5. Tested (mock) user role creation on a tenants-api client.
    6. Added new user management functionality to tenants-api client
    7. Tested (mock) new user management functionality.
    8. Renamed a few variables in a tenants-api client as per review comments.
    9. Added user and role deletion functionality to tenants-api client.
    10. Tested (mock) deletion functionality of 9.
    11. Renamed tenants-api to tenants_api in a metatests package.
    12. Fixed all issues as per review comments.
    13. Made a small change on role models

Change-Id: I554d5599bffdfc315d773ddc0f88c6e2d0fd4d59
Merge "Tenants API User model"