Install debian and python dependencies for LMS/CMS

* Create a global variable lms_source_repo to make it easy to update
  where to pull mitx from when mitx moves
* Create global variable venv_base to store location of edx virtual
  environment, defaults to /opt/edx
* Create edx virtual environment to store python modules
* Add tags to LMS and CMS tasks to make selective running and testing
  easier
* Install a long, static list of debian packages, which has been
  manually curated from other edx configurations; this probably needs
  more attention and in the future should probably be divided better
  among various roles.
* Install the list of python packages distributed with the mitx
  repository
* Install some really useful software, namely mosh and tmux