Restore compatibility with knife-windows

knife-windows bootstrap context inherits from Chef::Knife::Core::BootstrapContext
and uses the Chef 11 arity in its constructor (https://github.com/opscode/knife-windows/blob/master/lib/chef/knife/core/windows_bootstrap_context.rb#L36)

In order to protect compatibility with knife-windows make sure that we have a default
for the new parameter that is added to the BootstrapContext constructor.

/cc: @opscode/client-engineers