Fix ostestr invocation

More-recent versions of stestr are rather picky about wanting an integer
for concurrency [1], but ostestr prior to [2] defaults it to None. As a
result, all our func tests would fail with:

   TypeError: int() argument must be a string or a number, not 'NoneType'

Also, move the rolling upgrade job to non-voting since it's going to
pull in the old version but try to run with new deps.

[1] https://github.com/mtreinish/stestr/commit/c93bc81
[2] https://review.openstack.org/#/c/651275/

Change-Id: Icba6c46bcff0e0516107e0f6742d32c92488ae20

.functests

@@ -7,7 +7,7 @@ SRC_DIR=$(python -c "import os; print os.path.dirname(os.path.realpath('$0'))")
 
 cd ${SRC_DIR} > /dev/null
 export TESTS_DIR=${SRC_DIR}/test/functional
-ARGS="--serial ${@:-"--pretty"}"
+ARGS="--concurrency 1 ${@:-"--pretty"}"
 ostestr $ARGS
 rvalue=$?
 cd -  > /dev/null

.zuul.yaml

@@ -401,6 +401,7 @@
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/probe/.*$
               - ^(.gitreview|.mailmap|AUTHORS|CHANGELOG)$
+            voting: false
     gate:
       jobs:
         # For gate jobs, err towards running more jobs (so, generally avoid