Added config variable for External Network type in ML2

[OpenStack Infra]

https://review.openstack.org/149546
commit 26b4e57858ef83ef9343f053c2835a95f6e6c860
Author: Aman Kumar <email address hidden>
Date: Fri Jan 23 01:34:00 2015 -0800

    Added config variable for External Network type in ML2

    Description:
    With the ML2 Plugin, every network created has segments with
    provider:network_types being tenant_network_types.
    When applied to external networks, the types that could be in
    tenant_network_types parameter (like vxlan or gre) are not appropriate.

    Implementation:
    Added new config variable 'external_network_type' in ml2_conf.ini
    which contains the default network type for external networks
    when no provider attributes are specified, by default it is None.

    It also includes small code re-factoring/renaming of import statement.

    DocImpact

    Closes-Bug: #1328991

    Co-Authored-By: Romil Gupta <email address hidden>

    Change-Id: Idbbe6bced73cfedbe0f8e7abba35f87589b1a004

[Tom Fifield (fifieldt)]

https://review.openstack.org/#/c/177283/