Release candidate updates for Networking chapter

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

1) Moved Neutron ML2 sections before OVS sections and updated
   associated notes to steer users toward ML2.
2) Removed database population steps because Neutron populates the
   database at first run.
3) Moved 'enable_security_group' key to [securitygroup] section.
4) Removed extraneous colons from procedure titles.
5) Added command output to Neutron initial networks section.
6) Added command output to Nova initial networks section.

Change-Id: Ie677d199d2c64ef2a564eaa551295e1a321db02c
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements
Merge "Release candidate updates for Networking chapter"