Auto install all Python packages in src/edxapp-pip

[OmarIthawi]

Because of the statelessness of Docker containers, the devstack will forget any
pip install you'd do after restart.

Knowing that most of the packages like Figures and Course Access Groups are normally
need to survive such restarts, this feature only works on LMS/Studio.

The $ make tahoe.provision command will install all packages located inside
the src/edxapp-pip directory. There's no need to run this command manually as it will
run on every $ make dev.up.

For example to make Course Access Groups installed on every devstack startup:

$ cd devstack/../src/edxapp-pip/
$ git clone git@github.com:appsembler/course-access-groups.git
$ cd ../../devstack
$ make dev.up  # Now the repository will always be installed on every startup