Default value for horta logs synced folder.

If the `HORTA_LOGS` environment variable is not defined Vagrant shows this:

    Bringing machine 'default' up with 'virtualbox' provider...
    /usr/share/vagrant/plugins/kernel_v2/config/vm.rb:418:in `initialize': can't convert nil into String (TypeError)
    	from /usr/share/vagrant/plugins/kernel_v2/config/vm.rb:418:in `new'
    	from /usr/share/vagrant/plugins/kernel_v2/config/vm.rb:418:in `block in validate'
    	from /usr/share/vagrant/plugins/kernel_v2/config/vm.rb:413:in `each'
    	from /usr/share/vagrant/plugins/kernel_v2/config/vm.rb:413:in `validate'
    	from /usr/lib/ruby/vendor_ruby/vagrant/config/v2/root.rb:67:in `block in validate'
    	from /usr/lib/ruby/vendor_ruby/vagrant/config/v2/root.rb:63:in `each'
    	from /usr/lib/ruby/vendor_ruby/vagrant/config/v2/root.rb:63:in `validate'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/builtin/config_validate.rb:15:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/warden.rb:34:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/builtin/call.rb:57:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/warden.rb:34:in `call'
    	from /usr/share/vagrant/plugins/providers/virtualbox/action/check_virtualbox.rb:17:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/warden.rb:34:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/builder.rb:116:in `call'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:69:in `block in run'
    	from /usr/lib/ruby/vendor_ruby/vagrant/util/busy.rb:19:in `busy'
    	from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:69:in `run'
    	from /usr/lib/ruby/vendor_ruby/vagrant/machine.rb:147:in `action'
    	from /usr/lib/ruby/vendor_ruby/vagrant/batch_action.rb:63:in `block (2 levels) in run'

I think there should be an option to configure it later and start the vm anyway.