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
generator: use EXTRA_* env vars in the bash script

It's a bit odd to source a file (intended to contain env vars) from
the bash script and then for the python module be the one to actually
use the env vars, especially since the python module has command line
arguments.

Instead, lets read the values from the env vars in the bash script and
pass them to the python via command line arguments.

This is a slightly incompatible change - the env var was expected to
be comma separated before, but now its $IFS separated. However, none
of the users of this rc file actually have multiple values listed
in EXTRA_MODULES.

Change-Id: I79ce84e17c768daf13a7c8a3366e658ad10999b7
Merge "generator: add an EXTRA_LIBRARIES env variable"
Merge "generator: use EXTRA_* env vars in the bash script"