Support for flex allocation model in org vdc creation

[rana-Alotaibi1]

To help us process your pull request efficiently, please include:

• the changes will allow creating org vdc with flex allocation model and enable the elasticity and memory overhead options

• is_elastic and include_memory_overhead parameters are added to create_org_vdc

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This change is 

[rana-Alotaibi1]

@rocknes can you plz check this PR

[Etibru]

Update of the issue? The projet pyvcloud is end? @rocknes