Automate host configuration for functional testing

Automate host configuration for functional testing

This change adds a new script, configure_for_func_testing.sh, that
automates configuration of a host to support functional testing.  The
script's functionality is consumed by a refactored version of
gate_hook.sh, and both minimizes runtime and removes the previous
dependency on the devstack-gate repo.

Additionally, the dsvm-functional tox env is no longer dependent on
devstack to deploy neutron's rootwrap configuration system-wide.
Rootwrap configuration is now deployed to the target tox venv on each
tox invocation.

Change-Id: Iaf43be458bdf3c4535f95ee5a3a3b47a744020a0