Step: `apt`
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Install packages using apt, which needs to already have been
installed.

Step keys:

* `apt` &mdash; REQUIRED; value MUST be `install`.

* `tag` &mdash; REQUIRED; value is the tag for the root filesystem.

* `packages` &mdash; REQUIRED; value is a list of packages to install.

Example (in the .vmdb file):

    - apt: install
      tag: root
      packages:
      - python
      - linux-image-amd64
