Updated architecture chapter

As part of the installation guide improvement project, I performed
the following operations on the architecture chapter:

1) Added new diagrams for example architectures supported by the
   the installation guide.
2) Increased detail of node function descriptions.
3) Removed potentially confusing and overwhelming logical
   architecture diagram.
4) Clarified phrasing and wording.

The contents of this chapter rely on other portions of the guide
subject to change during this project and may need additional
updates.

Change-Id: I9b887f49a7d3ec94efc748a9056ec3a643ac7899
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements
Add command 'source openrc'

The command: 'glance image-create --name="CirrOS 0.3.1"
--disk-format=qcow2 --container-format=bare --is-public=true <
cirros-0.3.1-x86_64-disk.img' is only usable after the user has been
authenticated.
The command "source openrc" is executed to authenticate the user.

Closes-Bug: #1254084
Change-Id: I2cedb8e9adf372493f0f4a049805003880056d98
Merge "Updated architecture chapter"
Update flags and tables for neutron

Remove unneeded xi:include (metadata agent is not a plug-in)

Provide tables for new plugins and ML2 mechanisms

To avoid breaking the large tables appearance by using xrefs, the
bigswitch table is generated twice (one table for the ML2 mechanism, one
table for the standalone plugin). This is a (hopefully) temporary
change. Tables generations will probably be updated in Juno.

Change-Id: I27573975c83c5b25682135a4ea3ab852fec170ea
Closes-Bug: #1291785
Closes-Bug: #1292317
Closes-Bug: #1292318
Closes-Bug: #1294402
Closes-Bug: #1294405
Closes-Bug: #1294961
Merge "Update flags and tables for neutron"