An editable installation allows changes to be made to the source code
directly, and have those changes applied immediately without having to
reinstall.

    pip install -e /path/to/kolla-ansible

Above is currently working only in virtualenv, but there is no reason to
not allow in all cases. This is usefull for example when user is
building his own docker container with editable kolla-ansible installed
from git without virtualenv.

Change-Id: I185f7c09c3f026fd6926a26001393f066ff1860d