[python] allow adding trusted CA certs when using JWT

This patch updates Kudu Python client to allow for adding trusted
CA certificates when authenticating to Kudu servers via JWT.

TestJwt was correspondingly updated and re-enabled.

Change-Id: Ie13ea1877e7711d00ede41d141c01a5240bb9205
Reviewed-on: http://gerrit.cloudera.org:8080/19913
Tested-by: Kudu Jenkins
Reviewed-by: Marton Greber <greber.mx@gmail.com>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
5 files changed