Cleanup and fixes to build box tasks, and util module

- Move sysctl and file_update to util module
- Extend file_update to include a prefix param to look for matching
  lines to modify, or append if not found
- Fix ntpd package name to ntp (for EL6)
- Fix issue with push_key due to unreleased bugfix in
  Fabric.contrib.files, which didn't deal well with filenames containing
  a tilde
Fix incorrect arg ordering in util.service
Use sudo in util.yum and pep8ify the EPEL URL