Improve install guide neutron ML2 sections

I improved the neutron ML2 sections of the installation guide
as follows:

1) Removed 'openstack-config' steps.
2) Broke Debian steps into separate procedures for easier
   management.
3) Changed 'core_plugin' and 'service_plugins' keys in neutron.conf
   to use more compatible long values.

Please keep reviews in scope. Other patches depend on this patch.

Change-Id: Ia30d6656de0a0e44d53b12a0a044eeda87b3c873
Implements: blueprint installation-guide-improvements
Merge "Improve install guide neutron ML2 sections"