Add a means of listing installed software versions

We add a YAML-driven way to list installed software versions
at the end of the template createion.
The commands to list the versions of the desired software
will be indicated in each worker.*.yml file as the
'list_versions_by' array.

In general, this array will be constructed by listing
apt packages, and using appropriate shell commands.
List software verions for standard image

* We want versions of the installed packages, so use 'dpkg --list'
* Some commands are installed only for 'travis' user
  (Ideally, we should parametrize the user name)
* Some commands are massaged so that the output is of the form
  "SOFTWARE_NAME VERSION"
List software versions for ruby templates