Added "active" column on deployment list

When running 'rally deployment list' curent active deployment is being
higlighted by a "*" on a new column called "active".  Active deployment id is
comming from envutils._default_deployment_id() so this function is now public.

Change-Id: Ib911f10ee320d1ae6536f69587361fe1e35d8208
Trello: https://trello.com/c/d4wMvMsl