Redefine behavior for NvpAdvancedPlugin during network creation

When using the NvpAdvancedPlugin, *all* logical switches should
be created through VCNS. VCNS will then proxy the requests to
NVP. This patch implements such a behavior. This is achieved by
replacing the use of nvplib.create_lswitch with the one of the
vcns driver, as the remaining logic can be left as it is.

Fixes bug #1225011

Change-Id: I6c276f747d02ac0d4934c38bf2858cf9b73c5e24
Merge "Redefine behavior for NvpAdvancedPlugin during network creation"