provision/docker: fix database name in tests

Ops!
provision/docker: refactor the package

A lot of changes:

  * storing the type "container" in the database, instead of
    provision.Unit
  * simplify some tests
provision/docker: just remove containers, stopping them is not necessary

Closes #415.
provision/docker: remove the function parameter from newContainer

We can add it back when we need.
provision/docker: added function to list all containers from an app