Add config support for default network driver per COE

[Dane LeBlanc]

Magnum should provide support for configuration of default
network drivers per COE. An example use case could be
a cloud operator who would like to have control of what
default container network driver should be used on a
per-COE basis. The cloud operator in this case should
be able to set the default network driver per COE in
/etc/magnum/magnum.conf.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/237749

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/237749
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=471aa924fc60cea79e15cf54c7f707cd84969cbf
Submitter: Jenkins
Branch: master

commit 471aa924fc60cea79e15cf54c7f707cd84969cbf
Author: Dane LeBlanc <email address hidden>
Date: Thu Nov 5 12:39:07 2015 -0500

    Add Magnum config for default network driver per COE

    Problem description:
    Magnum should provide support for configuration of default network drivers per
    COE. An example use case could be a cloud operator who would like to have
    control of what default container network driver should be used on a per-COE
    basis. The cloud operator in this case should be able to set the default
    network driver per COE in /etc/magnum/magnum.conf.

    Proposed fix:
    Add 3 configuration list options to the config in /etc/magnum/magnum.conf:
    - kubernetes_default_network_driver
      (default='flannel')
    - swarm_default_network_driver
      (default='docker')
    - mesos_default_network_driver
      (default='docker')
    When a baymodel is created and the --network-driver attribute is not supplied,
    then the configured default network driver for the selected COE will first be
    checked against the list of allowed network drivers for that COE, and if
    allowed, the configured default network driver will be used for the new
    baymodel.

    Change-Id: I1e1d68c3348f609975b914926994bd79c1ef6d28
    Closes-Bug: #1504664