Allow pyvcloud to interact with stretched networks.

[exp-hc]

#625

• This allow interacting with Stretched Networks inside Datacenter Groups

• We've been using this for a significant amount of time internally without issue. To test, you'll need a Stretched Network and Datacenter Group.

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This change is 

[exp-hc]

@rocknes this is my first time using Reviewable, please let me know if you need anything else.

[exp-hc]

@rocknes any updates on this PR?