check explicitly if hostmanager.ignore_private_ip is set to true

As `vagrant hostmanager` (in contrary to eg. `vagrant provision`) doesn't implicate config finalization and validation, hostmanager.ignore_private_ip is never set to false in such a case. It is just an object
```ruby
class Config
# [...]
  UNSET_VALUE = Object.new
```
and the below statement
```ruby
unless machine.config.hostmanager.ignore_private_ip
```
results in `false`, causing the plugin to ignore private ip when the option is not set at all.

It would be probably better to force `vagrant hostmanager` to `finalize!` and `validate` the configuration, but I couldn't figure it out yet.