Decouple inventory_path and inventory generation

Supports the use case where data from outside of Vagrant (hosts and vars) are desired in addition to the inventory file that vagrant generates.

Previously, if the `inventory_path` was set, the vagrant inventory file would not be generated. This introduces the `generate_inventory` flag to decouple the `inventory_path` from the inventory generation process, allowing user inventories to be merged with the vagrant generated inventory.

If the `generate_inventory` is set to `true` and an `inventory_path` is set and is a directory, the generated inventory file will be placed in that directory and that directory will be passed to ansible.

If the `generate_inventory` flag is not set, previous behavior is maintained, eg. the inventory will not be generated if `inventory_path` is set.

TODO: Update Docs to reflect new functionality

CC: @gildegoma