NSX: lift restriction on DVR update

The restriction in place in the code is not justified considered
the capabilities of the DVR extension and the NSX backend.

Transformations of centralized routers into distributed should be
allowed.

Change-Id: I099b90d39247bc9a7adfb87344d77ccd8acfad9e
Closes-Bug: #1357048
Merge "NSX: lift restriction on DVR update"