Add a gate-specific tox env for functional tests

Add a gate-specific tox env for functional tests

The functional gate jobs (*-neutron-dsvm-functional) need special
configuration (e.g. OS_SUDO_TESTING, OS_ROOTWRAP_CMD) to run tests.
This patch adds a new gate-specific tox env (tox -e dsvm-functional)
that sets the appropriate values and allows easy duplication of the
results of the upstream job on a host that has successfully run
devstack with neutron enabled.

A new configuration option, OS_FAIL_ON_MISSING_DEPS, is also set by
the dsvm-functional tox env to ensure that dependency failures will
result in failures rather than skips when run in the gate.

Change-Id: Idbfef1ba72ed129a3021509c7969e2685b8a6d09
Partial-bug: #1336172