Move more paths into project_settings.py

deploy_dir, vcs_root, django_dir and ve_dir are all now defined in
project_settings.py and used by others.

ve_mgr.py will now use the requirements_file and ve_dir from
project_settings.py if they are not passed to it - and they are not by
default any more.

Finally this is all now working with the local/dev set up (ie fabric not
tested yet)