Allow configuring output .box file name

[stephenreay]

Description

vagrant package has a --output NAME option that allows setting the name of the output .box file.

Currently the Vagrant plugin for Packer hard-codes this to package.box

Use Case(s)

This will simplify the amount of out-of-packer processing required, when using the vagrant builder.

Potential configuration

{
    "output_name": "my-name-{{ user `version` }}.box"
}

Potential References

the current implementation