Add DistributedVirtualPortgroup constructor
Fix collapse_ws helper
Add vcsim_env helper

Tests can use this helper to optionally run against vcsim.
Add NetworkReference interface

This interface currently has 1 method to get the
VirtualDeviceBackingInfo, implemented by the Network and
DistributedVirtualPortgroup types.
Add network bats test
Change network flag to use NetworkReference
Merge pull request #150 from dougm/device-helpers

Add NetworkReference interface