Make predefined_networks parameter optional

[OpenStack Infra]

https://review.openstack.org/200530
commit f093b603d7e5c31d40c26329e8e5b99e1063b61b
Author: Oleg Gelbukh <email address hidden>
Date: Fri Jul 10 08:37:54 2015 +0000

    Make predefined_networks parameter optional

    Only create predefined networks in Neutron if 'predefined_network' attribute in
    clster settings is present and not empty. Otherwise, skip this part of manifest.

    So, if you need to skip creation of predefined networks, delete this parameter
    or leave it empty in deployment settings.

    It is important to note that if user disabled creation of predefined networks,
    some components that depend on them might fail to install. The list
    (non-comprehensive) includes:

    * Murano
    * Sahara
    * Mellanox plugin for Neutron

    Other services might be impacted as well.

    Change-Id: Iceed16072628cced6de9f8704b35f21fd4342e87
    Closes-bug: 1471244
    DocImpact