Fix functional CI job

The glanceclient-dsvm-functional job is currently running unit
tests, whereas it should be running the functional tests.  Fixed
by honoring the path in .stestr.conf like other trees.

Also removed tox.ini from the list of irrelevant-files for this
job, because as it turns out, it is relevant.

Change-Id: I59773caa00ff0dfc970c0e4d45aa5d8ae006b1c6

.stestr.conf

@@ -1,3 +1,3 @@
 [DEFAULT]
-test_path=./glanceclient/tests/unit
+test_path=${OS_TEST_PATH:-./glanceclient/tests/unit}
 top_path=./

.zuul.yaml

@@ -34,7 +34,6 @@
       - ^(test-|)requirements.txt$
       - ^lower-constraints.txt$
       - ^setup.cfg$
-      - ^tox.ini$
 
 - job:
     name: glanceclient-tox-keystone-tips-base