Make testr preserve existing OS_* env vars values

Values for OS_TEST_TIMEOUT, OS_STDOUT_CAPTURE and OS_STDERR_CAPTURE
environment variables are fixed in .testr.conf now. It should be possible
to override them for each tox/runtests.sh call or for the current shell
session without making changes to .testr.conf.

Fixes bug 1182416.

Change-Id: I6c7839fa78b53ac8ea358ce0143a00931da96a9e

.testr.conf

@@ -1,4 +1,8 @@
 [DEFAULT]
-test_command=OS_STDOUT_CAPTURE=1 OS_STDERR_CAPTURE=1 OS_TEST_TIMEOUT=60 ${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests $LISTOPT $IDOPTION
+test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
+             OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
+             OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
+             ${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests $LISTOPT $IDOPTION
+
 test_id_option=--load-list $IDFILE
 test_list_option=--list