Add hook scripts for the functional infra job

Infra jobs support hooks to customize behavior.  This change adds
scripts that are intended to be called by the neutron-dsvm-functional
jobs as hooks.   Defining hook behavior in-tree instead of directly in
the job definition has the advantage of allowing changes in hook
behavior to be tested before merge.

The gate_hook.sh script added by this change installs a rootwrap
filter to allow functional tests to destroy python processes that
they have spawned.  The tests use the python executable provided by
their tox env, and the default rootwrap configuration does not
support killing processes launched by non-system python executables.

This change will have to merge to master and stable/icehouse before
the supporting infra config change can be merged:

https://review.openstack.org/#/c/116458/

Partial-bug: #1360658
Change-Id: Idcc6195f91fac833c14892f27fb793048ef964b9