Renamed tox targets for functional with python3

The rename is needed to match what
gate-neutron-dsvm-functional-python35-ubuntu-xenial-nv currently calls.

Change-Id: Ibda203a20a17d7e16217b19e5b83b38741ffb834

tox.ini

@@ -42,7 +42,7 @@ deps =
   {[testenv]deps}
   -r{toxinidir}/neutron/tests/functional/requirements.txt
 
-[testenv:functional-py35]
+[testenv:functional-python35]
 basepython = python3.5
 setenv = {[testenv:functional]setenv}
 deps =
@@ -58,7 +58,7 @@ commands =
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}/etc {envdir}/bin
   {toxinidir}/tools/ostestr_compat_shim.sh {posargs}
 
-[testenv:dsvm-functional-py35]
+[testenv:dsvm-functional-python35]
 basepython = python3.5
 setenv = {[testenv:dsvm-functional]setenv}
 deps =