Jenkins config to speed up builds

Summary:
This doesn't quite contain all relevant changes, some snuck into a
previous config commit. However, it is most. What changed is:

1. deploy-daily now uses the same workspace as deploy-staging, so that
   there are smaller incremental changes to filemod_db files. The most
   recent staging build is likely close to the new deploy, and the last
   deploy was probably from a day ago. The "throttle" plugin prevents
   them from running in the workspace at the same time.
2. We set the current deploy-daily commit to be the master branch
   manually. Before, the git-plugin did this same thing but only after
   first iterating over all branches which took ~4 minutes.
3. website-commit and website-allcheck now run "make clean" instead of
   using the git-plugin's "Clean workspace" which ran "git clean -qfdx".
   The result is that node_modules/ is left in place across builds.

Test Plan: This is running on jenkins.khanacademy.org

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D5042