etcd3: Allow for multi-host deployments
etcd3: Allow for multi-host deployments

In Multi host deployments, it is possible to run ETCD in a different
host than the SERVICE_HOST (where all the controllers run). This patch
brings that distinction.

Change-Id: I15fe6f25eedf1efebaab81cce26b080577b856cc
Signed-off-by: Antoni Segura Puimedon <dc6adfafa1816104538e250c6bf1b7156f3887c2@celebdor.com>

group-policy/devstack,richliu/devstack,noironetworks/devstack,openstack-dev/devstack,group-policy/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack,richliu/devstack,noironetworks/devstack,dtroyer/devstack,dtroyer/devstack