Add the systemd-networkd to centos 7 installations

This adds the systemd-networkd package to the base image for centos 7.
This is being done so we're able to use unify container networking using
systemd built-ins instead of having to manage unique network scripts
across distros.

Change-Id: Ia1f8f3589d70c50813f85d0b1fe037225971bf6c
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>

vars/redhat-7.yml

@@ -88,6 +88,7 @@ _lxc_cache_distro_packages:
   - python2
   - rsync
   - sudo
+  - systemd-networkd
   - tar
   - wget
   - which