Correct issue with virtualenv rootwrap

Because we use rootwrap in a venv we need to update the exec_dirs.
Without doing this *some* commands will break for various reasons that
I won't get into in this review in detail. But this is required for
neutron-l3-agent in a container + drop-root.

Change-Id: I1a09f7188fdd501b7ce251d9f2fb0e5b10222142
Partially-Implements: blueprint drop-root