[docs]Document which neutron plugins/ML2 drivers are included into MOS iso

[Evgeniya Shumakher]

Our marketing team and partners are frequently asking questions like:
* are the Citrix NetScaler drivers included in MOS 6.0 release?
* is Brocade ML2 driver included into MOS?

We should explain our strategy here:
* What is included, what's not
* If i want to enable a networking backend other than OVS, what should i do
* if i want to enable LBaaS, VPNaaS what should i do (i believe the answer is plugins)

[Evgeniya Shumakher (eshumakher)]

Please reach out to Sergey Kolekonov to get information about Neutron

[Irina Povolotskaya (ipovolotskaya)]

As to LBaaS and VPNaaS, we clarify that within our plug-in-related doc issues.

[Meg McRoberts (dreidellhasa)]

Each plug-in/driver/technology should have an entry in the Terminology Reference that tells its MOS status and points to other documents (in or out of our doc set) for more information.

[Irina Povolotskaya (ipovolotskaya)]

I work on Terminology (VPNaaS, LBaaS are both for Neutron): https://review.openstack.org/#/c/140350/
As to clarifying what is included into MOS and what is not, could we pass this task to Evgeny?

I'll help him with :ref: to the docs on plug-ins.