Allow `--force-join` option to be configured via class parameter

  * Allow using an option to override client's entry and force enrollment

    + `ipa-client-install` can use "--force-join" option to override
    the host entry on the server and force client enrollment.

    + $freeipa::force_join sets $client_install_cmd_opts_force_join

      - it checks if you want to use that option or not;
      - should you do, we introduce it to $client_install_cmd

  * Present $client_install_cmd_opts_force_join to `ipa-client-install`

  * Referency information about "force_join" option and its default value

  * Document @force_join parameter and what the default behavior is

  * Officially introduce `@force_join` into a Puppet manifest