NetworkListing: vlan property not available for DVPortSetting

[timogoebel]

22a9988 added support for listing the vlan id of a network.

It uses network.config.defaultPortConfig.vlan to get the vlan id. That property is only available when network.config.defaultPortConfig is of kind VMwareDVSPortSetting and not available when it is a RbVmomi::VIM::DVPortSetting.

We should add a gate for that.

Stack trace:

 | NoMethodError: undefined method `vlan' for #<RbVmomi::VIM::DVPortSetting:0x007f576e0c79a8>
 | /usr/share/foreman/vendor/ruby/2.1.0/gems/fog-vsphere-2.1.0/lib/fog/vsphere/requests/compute/list_networks.rb:30:in `network_attributes'
 | /usr/share/foreman/vendor/ruby/2.1.0/gems/fog-vsphere-2.1.0/lib/fog/vsphere/requests/compute/list_networks.rb:12:in `block in list_networks'
 | /usr/share/foreman/vendor/ruby/2.1.0/gems/fog-vsphere-2.1.0/lib/fog/vsphere/requests/compute/list_networks.rb:10:in `map'
 | /usr/share/foreman/vendor/ruby/2.1.0/gems/fog-vsphere-2.1.0/lib/fog/vsphere/requests/compute/list_networks.rb:10:in `list_networks'
 | /usr/share/foreman/vendor/ruby/2.1.0/gems/fog-vsphere-2.1.0/lib/fog/vsphere/models/compute/networks.rb:12:in `all'

Initially reported on the Foreman Community.