Use Pipfile.lock only in tox gates

Pipenv validates that the Pipfile.lock that it uses for installing
packages is not out of date before following through on execution. If it
is not, it will throw an error. This change adds an env that tells
Pipenv to ignore the Pipfile and only use Pipfile.lock so gates do not
fail due to out of date packages.

Change-Id: I2fa541b31e14561c7cefc330e7ef2cc0b0e4abc6

tox.ini

@@ -10,6 +10,7 @@ setenv =
   LANGUAGE=en_US
   LC_ALL=en_US.utf-8
   PIPENV_VERBOSITY=-1
+  PIPENV_IGNORE_PIPFILE=1
 deps =
   pipenv
 passenv = http_proxy https_proxy HTTP_PROXY HTTPS_PROXY no_proxy NO_PROXY PBR_VERSION