Add support for YAML lists to apt module.

This adds support for native YAML lists to the `package` parameter of the `apt` module.

For example, it becomes possible to use this:

    - name: install Postfix
      apt:
        package:
          - postfix
          - postfix-cdb
          - postfix-mysql
          - postfix-ldap
        state: latest

Instead of this:

    - name: install Postfix
      apt:
        package: postfix,postfix-cdb,postfix-mysql,postfix-ldap
        state: latest