Use rsync over ssh to speed up Jenkins config sync

Summary:
Previously this would establish an SSH connection for each file. Now,
only a single SSH connection is created, and rsync is used to only copy
the parts of the destination files that are changed. Much, much faster
than before.

Test Plan:
I made a config change to website-commit on jenkins.khanacademy.org then
ran ./fetch_jenkins_config.sh and verified that the local config was
similarly modified.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2542