Parameterizing edx git repository location

* Creates global variable $platform_code_dir to store
  /opt/wwc/edx-platform, to replace the semihardcoded /opt/wwc/mitx that
  we had before.
* Plugs $platform_code_dir in a lot of different places
* Templatize sef's update script hack
Tag tasks update or install

* update tasks are meant to be the fast ones and the ones we want to be
  sure to always do (directory creations, permissions, etc.)
* install tasks are all the really slow things, like the git repo clone
  and package installations
* Every task can by run by --tags=install,update - which is the same as
  having no tags at all.