docs: merge multiple interface sections with provider network section
docs: merge multiple interface sections with provider network section

In this guide, multiple interfaces in DevStack is only used when doing
provider networking, so let's go ahead and just put the information
inside the provider network section. That way it won't be confusing.

Change-Id: I66f58ffb936230e72ac4cf8c04668e25dac5b17a
thomasem/devstack,richliu/devstack,nawawi/openstack,dtroyer/devstack,noironetworks/devstack,mssumanth/devstack,bigswitch/devstack,pombredanne/devstack,varunarya10/devstack,group-policy/devstack,bswartz/devstack,richliu/devstack,Mahito/devstack,openstack-dev/devstack,samgoon/devstack,dtroyer/devstack,prithivm/branch01,avvocatodemarchis/devstack,mssumanth/devstack,dirkmueller/devstack,openstack-dev/devstack,bigswitch/devstack,prithivm/branch01,bswartz/devstack,avvocatodemarchis/devstack,pombredanne/devstack,group-policy/devstack,nawawi/openstack,varunarya10/devstack,thomasem/devstack,noironetworks/devstack,samgoon/devstack,dirkmueller/devstack,Mahito/devstack