Add dhcp-all-interfaces.target for syncing units

Currently there is no way for a service to become aware that
dhcp-all-interfaces is finished configuring all the interfaces at
boot time. This causes problems for applications like the
ironic-python-agent which scans the interfaces when it first starts as
part of the inspection stage and can race against dhcp-all-interfaces
bringing up the interfaces, leading to inconsistent results.

This patch ensures that the dhcp-all-interfaces script runs before any
network interface is configured and brought up by the rest of the
system, and also ensures that the ironic-agent element also waits for
the network to be online before starting. This is done by using the
network targets provided by systemd.

Change-Id: Id9583b7f54361aa603a6229da598ad6a0f0f7938

elements/dhcp-all-interfaces/install.d/dhcp-interface@.service

@@ -1,6 +1,8 @@
 [Unit]
 Description=DHCP interface %I
-After=network.service network.target
+After=network.target
+Before=network-pre.target
+Wants=network-pre.target
 
 ConditionPathExists=!/etc/sysconfig/network-scripts/ifcfg-%I
 

elements/ironic-agent/install.d/ironic-agent-source-install/ironic-python-agent.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Ironic Python Agent
-After=network.target
+After=network-online.target
 
 [Service]
 ExecStart=/usr/local/bin/ironic-python-agent