remove_vehicle function in services/director.py

[tanishqjasoria]

Purpose of the PR:

To resolve #59: Add method to remove vehicles from Director repo

Summary of Changes:

1. services/inventorydb.py
   added a method deregister_vehicle
2. services/director.py
   added method remove_vehicle
   added method remove_director_repo_for_vehicle

[mnm678]

@tanishqjasoria Can you add some test cases for this?

[tanishqjasoria]

@mnm678 I have added a test for remove_director function!

[tanishqjasoria]

While registering a vehicle with the director using the add_new_vehicle method, while creating the repository for the vehicle, the current working directory(which is the uptane working directory) is changed to the director repos directory (as the repository Tool expects to use the current directory)
Therefore after adding a vehicle, it needs to be changed to the uptane working directory again.