generator: add an EXTRA_LIBRARIES env variable

If Nova uses oslo.messaging, we want to somehow configure the generator
to automatically add options from oslo.messaging. Perhaps the easiest
and most obvious way of doing that would be to add:

  export NOVA_CONFIG_GENERATOR_EXTRA_LIBRARIES=oslo.messaging

to oslo.config.generator.rc.

Change-Id: I977eb7009d677430a20987e31dcaeea3ace0fc6a
Merge "generator: add an EXTRA_LIBRARIES env variable"