nuageinit: add a post network script

refactor nuageinit to allow a 3rd execution point during boot:

1. nuageinit is invoked before NETWORKING with a minimalistic network setup for openstrack and potentially other network config setup. it tries to configure everything which is not requiring any network.
2. nuageinit is invoked again post NETWORKING but pre SERVERS, in the phase it does all that requires network, like dealing with packages. Note that creating users have been moved to this phase to allow the installation of shells like bash or zsh prior the creation of the users, before that the user creation was failing if a non installed shell was requested.
3. nuageinit will execute at the rc.local time all the specified scripts and commands.

MFC After: 1 week

(cherry picked from commit 2a05d577ab30dcefcf51def4c65a64af30610c85)