Remove some readonly attributes since they need to be included in the Put request made

[vinayada1]

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[sergey-shandar]

@ravbhatnagar changes in a stable Network API spec.

[ravbhatnagar]

Signing off from ARM side. I dont think this is a breaking change. @sergey-shandar - Let me know if I am missing anything on this on why do you think this is a breaking change. Earlier, this property was not settable by the clients, but now it is. Existing clients will/should continue to work.