doc: update the security groups admin doc
doc: update the security groups admin doc

This makes the following changes:

* re-orders the page to move the nova-network
  specific information to the bottom
* creates two sections: one for CLI and one for
  nova-network
* mentions at the top that by default neutron
  manages security groups and their quota and
  links to the neutron docs
* drops the mention of the 'nova' CLI since there
  are no examples in this doc using that CLI

Change-Id: Ifd23424ac14bacf4bf7a0716c268f48ec869a41e

openstack/nova,klmitch/nova,mahak/nova,openstack/nova,rahulunair/nova,klmitch/nova,rahulunair/nova,klmitch/nova,klmitch/nova,rahulunair/nova,mahak/nova,openstack/nova,mahak/nova