Add support for allowable network drivers configuration

[Dane LeBlanc]

Magnum should provide support for configuration of what
container network drivers are allowed, and this needs to
be supported on a per-COE basis.

With the changes proposed in
https://review.openstack.org/230147, the allowable
network drivers per COE are hardcoded. Magnum should
allow for a cloud operator to override these hardcoded,
allowed network drivers via configuration in
/etc/magnum/magnum.conf.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/233275

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/233275
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=ce0ed4e44dc398455b9904ada78cbf5f094d7e80
Submitter: Jenkins
Branch: master

commit ce0ed4e44dc398455b9904ada78cbf5f094d7e80
Author: Dane LeBlanc <email address hidden>
Date: Fri Oct 9 16:42:45 2015 -0400

    Add support for allowable network drivers configuration

    Problem:
    With the changes that were merged with https://review.openstack.org/230147, the
    choices of container network drivers that can be selected for a given COE type
    when creating a Magnum baymodel are hardcoded, and are based on network drivers
    that Magnum can support for each COE.

    These hard-coded selections will probably make sense in most cases. However, in
    some cloud instances, a cloud provider or cloud admin may want to restrict the
    choices of network drivers that a user can select for a given COE even further,
    based on specific restrictions in the cloud.

    Magnum should provide support for configuration of what container network
    drivers are allowed, and this needs to be provided on a per-COE basis.

    Proposed fix:
    Add 3 configuration list options to the config in /etc/magnum/magnum.conf for
    configuring allowed nework drivers for kubernetes, swarm and mesos
    respectively. The keyword 'all' can be used to allow all network drivers
    supported by Magnum for that COE. The new config options are:
    - kubernetes_allowed_network_drivers
      (default=['all'])
    - swarm_allowed_network_drivers
      (default=['all'])
    - mesos_allowed_network_drivers
      (default=['all'])
    Validation of the --network-driver attributes for a baymodel create will be
    performed against this above configuration on a per-COE basis.

    Change-Id: Ibaa21d43fd6b5c1a6acc10e56145280eeaea8534
    Closes-Bug: #1504635