NO-JIRA: Fix problems with system tests.

- Skip system_tests_broker if its requirements (qpidd etc.) are not installed.
- Fix logic for creating/deleting test directories, was failing on old python.

git-svn-id: https://svn.apache.org/repos/asf/qpid/dispatch/trunk@1606938 13f79535-47bb-0310-9956-ffa450edef68