provision/docker: add functions that will be used to serialize keys

Related to #850.
provision/docker: starting to support the SSHHostPort attr

- now networkInfo returns a containerNetworkInfo (it's better than
  returning another string and allowing us to break everything just
  because we changed two ports)
- add SSHHostPort and PrivateKey fields to the container type. At the
  moment, we're not using the PrivateKey field yet
- map the 22/tcp port for the container (and capture it in the
  SSHHostPort)

We still need to define SSHHostPort in other method calls.

Related to #850.
provision/docker: set SSHHostPort on setNetworkInfo

Related to #850.
provision/docker: generate a rsa key pair for container

One step left: using the private key in the database to run commands in
the container! :)

Related to #850.
provision/docker: remove fs mocking

We don't need it here anymore :) Thanks to #850.
provision/docker: delete unused code