Issue 59 adding remove vehicle method

[mm7259]

Purpose of the PR:

Add remove vehicle method to Director class

Summary of Changes:

Added deregister_vehicle in Inventorydb class and delete_director_repo_for_vehicle in Director class, in order to implement the intended functionality

Further Requirements:

(If documentation or the demo needs to be changed due to this PR, has this
already been included in the PR? What still needs to be done?)

[coveralls]

Coverage increased (+0.06%) to 96.786% when pulling 9bdb741 on mm7259:issue_59_adding_remove_vehicle_method into 22efb0e on uptane:develop.