Add new env variable to specify test path

This commit adds the env variable OS_TEST_PATH to specify the test
discovery path for testr. This can be used to specify which path will
be used for test discovery.
Closes-Bug: #1260710

Change-Id: Idcc8f6b5d82a2adc1a1f8ad04a47e8d694dc7de7
diff --git a/.testr.conf b/.testr.conf
index 05b12c4..c25ebec 100644
--- a/.testr.conf
+++ b/.testr.conf
@@ -2,7 +2,8 @@
 test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
              OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
              OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
-             ${PYTHON:-python} -m subunit.run discover -t ./ ./tempest $LISTOPT $IDOPTION
+             OS_TEST_PATH=${OS_TEST_PATH:-./tempest} \
+             ${PYTHON:-python} -m subunit.run discover -t ./ $OS_TEST_PATH $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list
 group_regex=([^\.]*\.)*