Add NetworkManager and dhcp-client for CentOS 8

As described inline, NetworkManager and dhcp-client make up the basic
networking for centos 8 installs; bring them into the base image.

Although in infra we then use simple-init, some other users find this
helpful.

Change-Id: Ib9f32e73bf9109cc1b659fe1deceb1a15301ffeb

diskimage_builder/elements/yum-minimal/package-installs.yaml

@@ -11,3 +11,10 @@ linux-firmware:
     uninstall: true
 linux-firmware-whence:
     uninstall: true
+# NOTE(ianw) pkg-map restricts this to !rhel/centos 7; in 7
+# the "initscripts" package handles basic networking, in
+# 8+ it's the role of NetworkManager and dhcp-client.  This
+# mirrors the default packages installed in upstream cloud images
+# to facilitate a basic network.
+NetworkManager:
+dhcp-client:

diskimage_builder/elements/yum-minimal/pkg-map

@@ -1,4 +1,12 @@
 {
+  "release": {
+      "centos": {
+          "7": {
+              "NetworkManager": "",
+              "dhcp-client": ""
+          }
+      }
+  },
 "family": {
     "redhat": {
       "lsb_release": "redhat-lsb-core"