A Template unit always contains an id field. It should therefore inherit
UnitWithId instead of Unit, so that the `partial_id()` and `id()`
methods are inherited as well.

By default, templates using Jinja2 engine gets their id rendered as soon
as they get accessed, but since they don't have the appropriate
environment, the parameters are replaced with an empty string.

This is why when you run the command `checkbox-cli list template`, you
may see things like this:

    template 'com.canonical.certification::camera/still_'

instead of

    template 'com.canonical.certification::camera/still_{{ name }}'

Note that this does not happen with standard Python formatted strings:

    template 'com.canonical.certification::camera/led_{name}'

In order to make the behavior consistent regardless of the template
engine used, this commit:

- introduces a unit property that always returns "template" (similar to
how the Job unit always returns "job").
- modifies Unit.get_record_value() to prevent rendering using Jinja2
templating if the unit is a Checkbox Template

It also contains unit tests for both the TemplateUnit and its Validator.

…d, if absent

PlainBoxObject is used when exposing Units at a high level; for
instance, when running `checkbox-cli list template`.

Instead of exposing the Template Unit's id field, expose the template-id
field.

In order to have more granularity on what is displayed for the template
id, use methods similar to the Job Unit.

This method overrides UnitWithId.explain() which displays the Unit's id.

In the case of a Template, this is misleading, and the template_id (or,
in this case, template_partial_id to prevent displaying the provider
namespace) should be used instead.

This is useful when reporting errors using `./manage.py validate`, for
instance.