Make mitx git checkout to /opt/wwc work

* create_users creates an 'edx' group
* create_users task edited to put all admin users in adm and edx groups,
* create_users assigns all users a shell
* lms installs ssh keys for mitx checkout, creates target dir in
  /opt/wwc, and checks out the mitx source
Cleanup for git workflow sharing
Configurable app base directory

* Replace hardcoded instances of /opt/wwc with $app_base_dir, which
  defaults to /opt/wwc.
* Add tags to nginx tasks so they can be selected independently.
Merge pull request #15 from edx/jrbl/feature/git_checkout

Jrbl/feature/git checkout