Add OS_TOP_LEVEL to testr conf

When installing tempest, it could be that the root of tempest code
is not a subfolder of the folder where the testrepository
configuration is held, and where test results are stored.
unittest2 will refuse to run tests for which the discovery root
is not a subfolder of the specified top level folder.

To fully support this scenario, adding OS_TOP_LEVEL in .testr.conf
and defaulting it to "./" which is the current value.

Change-Id: I60f788051d6a10ab00aecef5d09093a68f7f4030
diff --git a/.testr.conf b/.testr.conf
index 4f6e0b3..95a4fb4 100644
--- a/.testr.conf
+++ b/.testr.conf
@@ -3,7 +3,7 @@
              OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
              OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
              OS_TEST_LOCK_PATH=${OS_TEST_LOCK_PATH:-${TMPDIR:-'/tmp'}} \
-             ${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./tempest/test_discover} $LISTOPT $IDOPTION
+             ${PYTHON:-python} -m subunit.run discover -t ${OS_TOP_LEVEL:-./} ${OS_TEST_PATH:-./tempest/test_discover} $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list
 group_regex=([^\.]*\.)*