Disable urllib3 warnings

1. When running in compass OSA, lots of SubjectAltNameWarnings are
   shown in console, so just disable the warnings here.
   See https://github.com/shazow/urllib3/issues/497 for details
2. Redirect warnings issued by the warnings module to the logging system
   via logging.captureWarnings(True)
3. But urllib3 warning is still there for two cmds about rally/tempest.

Change-Id: Ic22cf8e2babc10c9d55a7ed46b841312a4e430a7
Signed-off-by: Linda Wang <wangwulin@huawei.com>