Fix CNA element ordering
Fix CNA element ordering

Element ordering for the CNA wrapper (ClientNetworkAdapter REST object)
was incorrect, resulting in unmarshalling errors if attempting to set
any of the fields ConfiguredMTU, OvsBridge, or OvsPortExternalIds if
they're missing.

The VirtualNetworks element should be last.  This change set fixes it.

Change-Id: I3133c73573f07228b9bb64a89e01c6310dcd628e
Closes-Bug: #1731657

powervm/pypowervm,pypowervm/pypowervm