kraken lock should prepare a virtualenv as specified in the lockfile

[daladim]

Kraken is lacking a lock command that re-creates the environment specified in the lockfile

This would make this workflow possible:

• clone a kraken-managed repo
• kraken lock # or any more suitable command name
• tada, a virtual env is setup, with everything specified in the lockfile, so that I can do a kraken run build that will work, even if the newest upstream releases of the dependencies have changed