Make sure the config validation does not fail if plugin configuration is not provided

Simply bringing up a plain old Vagrant machine failed if hostmanager
configuration was not provided. This was optional according to the readme. This
commit ensures that not having any configuration is a valid scenario.

Reviewed-by: Philippe Godin <godp1301@gmail.com>
Specifically use bash for test.sh

Reviewed-by: Philippe Godin <godp1301@gmail.com>
Sort the content of expected and actual host file in the tests.

We can't predict in which order vagrant-hostmanager handles the host file when appending to it.

Reviewed-by: Mathieu Mitchell <mmitchell@iweb.com>