Document need for extension driver for Provider Support

[Carol Bouchard]

Our documentation is lacking mention of needing an extension driver for Provider Support.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/603468

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: release-6
Review: https://review.openstack.org/603659

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/603468
Committed: https://git.openstack.org/cgit/openstack/networking-cisco/commit/?id=9e8731e8be9941d8f1b200db97e8a17481e91200
Submitter: Zuul
Branch: master

commit 9e8731e8be9941d8f1b200db97e8a17481e91200
Author: Carol Bouchard <email address hidden>
Date: Tue Sep 18 14:40:07 2018 -0400

    Add cisco providernet extension to Nexus doc

    The existing documentation is missing mention of the extension driver
    'cisco_providernet_ext'. This driver is responsible for identifying
    when a vlanid is that of a provider. Configuration options
    'provider_vlan_auto_create' and 'provider_vlan_auto_trunk'
    depend on this identification to determine whether or not to
    configure the vlan or apply vlan to an interface on the Nexus Switch.

    Change-Id: I04436fa1284c75bcc6e3615435f656a6834c9c06
    Closes-bug: #1793146

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/603659
Committed: https://git.openstack.org/cgit/openstack/networking-cisco/commit/?id=7f51936083ad7f29a9da03576ed3454ffc8cecf0
Submitter: Zuul
Branch: release-6

commit 7f51936083ad7f29a9da03576ed3454ffc8cecf0
Author: Carol Bouchard <email address hidden>
Date: Tue Sep 18 14:40:07 2018 -0400

    Add cisco providernet extension to Nexus doc

    The existing documentation is missing mention of the extension driver
    'cisco_providernet_ext'. This driver is responsible for identifying
    when a vlanid is that of a provider. Configuration options
    'provider_vlan_auto_create' and 'provider_vlan_auto_trunk'
    depend on this identification to determine whether or not to
    configure the vlan or apply vlan to an interface on the Nexus Switch.

    Change-Id: I04436fa1284c75bcc6e3615435f656a6834c9c06
    Closes-bug: #1793146