devenv up and devenv shell should do equivalent things on their first runs

[mcdonc]

python-rewrite branch.

Currently, if you check out a project and do a bare "devenv up", it doesn't do anything except return. You first must run devenv shel at least once to get deps installed.

IMO, the commands should invoke the same codepath, with one leading to a shell, and one leading to running processes / services.