The most simple way RDO Juno All-in-one install on CentOS 7 via packstack
Instructions here: https://openstack.redhat.com/Quickstart

For now one NIC is enough. Packstack will do all job for you, but 
When it's done you have manually create  ifcfg-br-ex for OVS bridge br-ex,
ifcfg-em1 ( or whatever you had originally ) as OVS port for br-ex.
Enable service network && disable NetworkManager ( it's a must ).

Samples of ifcfg-* files

[root@juno1 network-scripts(keystone_admin)]# cat ifcfg-br-ex
DEVICE="br-ex"
BOOTPROTO="static"
IPADDR="192.168.1.127"
NETMASK="255.255.255.0"
DNS1="83.221.202.254"
BROADCAST="192.168.1.255"
GATEWAY="192.168.1.1"
NM_CONTROLLED="no"
DEFROUTE="yes"
IPV4_FAILURE_FATAL="yes"
IPV6INIT=no
ONBOOT="yes"
TYPE="OVSIntPort"
OVS_BRIDGE=br-ex
DEVICETYPE="ovs"

[root@juno1 network-scripts(keystone_admin)]# cat ifcfg-enp2s0
DEVICE="enp2s0"
# HWADDR=00:22:15:63:E4:E2
ONBOOT="yes"
TYPE="OVSPort"
DEVICETYPE="ovs"
OVS_BRIDGE=br-ex
NM_CONTROLLED=no
IPV6INIT=no