provision/docker: add method to open a shell to the container

We still have some problems with the allocated pty, but we're getting
there.

Related to #853.
provision/docker: move code that wait for the ssh server

It's better to wait for it after creating it :)
provision/docker: add handler for opening ssh

Missing the command, and we also need improvements in the way we
connect to the server (I'm seriously considering using os/exec here,
just like Juju does).

Related to #853.
queue: less logging

Let's not be that verbose. Related to #856.
queue: less logging

Closes #856.