Add OS_TEST_PATH to testr

Change default test path to unit tests, and support setting
$OS_TEST_PATH to specify a different path (such as functional).

Partial-Bug: #1446442

Change-Id: I20461e1867f2c6e1e1432ab79fced82e8c0b4cc9

.testr.conf

@@ -1,4 +1,4 @@
 [DEFAULT]
-test_command=${PYTHON:-python} -m subunit.run discover -t ./ . $LISTOPT $IDOPTION
+test_command=${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./heatclient/tests/unit} $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list