Added Rex::Virtualization::VBox::bridge and Rex::Box::VBox.select_bridge

- Rex::Virtualization::VBox::bridge returns available bridged
interfaces.
- Rex::Box::VBox.select_bridge returns selected bridged interface name.
- Modified Rex::Box::VBox#import_vm to execute
Rex::Box::VBox.select_bridge when bridge-option is not specified.
Merge remote-tracking branch 'hayajo/feature/select_bridge' into 0.42

merged: #153