Don't initialize neutronv2 state at module import

Currently, neutronv2.API defines conductor_api when the module is
imported. This could easily happen before any configuration files are
parsed so e.g. use_local may not be applied.

In the case of oslo.messaging, I was seeing this cause problems bcause
it was happening before oslo.messaging transport was initialized.

blueprint: oslo-messaging
Change-Id: Ia193442d9c9156b1afdbb24be3c83f4be3c8f35f
Merge "Don't initialize neutronv2 state at module import"