(QENG-1225) Implement uber_build_with_polling.

Includes:
 * rspec tests for new jenkins utility methods
 * jenkins utility methods for grabbing information from given job url
 * `uber_build_with_polling` rake task that depends on `uber_build` then polls
   the resulting dynamic jenkins jobs and exits with nonzero exit status if any
   fail.

Signed-off-by: Wayne <wayne@puppetlabs.com>