[client] allow adding trusted CA certs for RPC connection negotiation

This patch updates Kudu C++ client API to allow for adding trusted
CA certificates into the TLS context of the client's messenger.
Doing so allows to negotiate an RPC connection with Kudu servers
when the client is using JWT for authentication.  Corresponding tests
have been updated as well.

Change-Id: I43a84a7a8e23556d85706b7714c3b40d8c568447
Reviewed-on: http://gerrit.cloudera.org:8080/19907
Tested-by: Kudu Jenkins
Reviewed-by: Zoltan Chovan <zchovan@cloudera.com>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
4 files changed