Add pre-install phase for extras.d plugins

An additional call hook for the extras.d plugins that is called before
any service installation occurs turns out to be needed by Ceph and
useful for other plugins.

This is called between the installation of the system packages listed
as prerequisites and the installation of the actual services.

Change-Id: Id723bdc7542182d2b2eff40c0dc0127307f094c5

extras.d/README.md

@@ -22,9 +22,24 @@ sourced with one or more arguments, the first of which defines the hook phase:
     stack: called by stack.sh.  There are four possible values for
         the second arg to distinguish the phase stack.sh is in:
 
-        arg 2:  install | post-config | extra | post-extra
+        arg 2:  pre-install | install | post-config | extra
 
     unstack: called by unstack.sh
 
     clean: called by clean.sh.  Remember, clean.sh also calls unstack.sh
         so that work need not be repeated.
+
+The `stack` phase sub-phases are called from `stack.sh` in the following places:
+
+    pre-install - After all system prerequisites have been installed but before any
+        DevStack-specific services are installed (including database and rpc).
+
+    install - After all OpenStack services have been installed and configured
+        but before any OpenStack services have been started.  Changes to OpenStack
+        service configurations should be done here.
+
+    post-config - After OpenStack services have been initialized but still before
+        they have been started. (This is probably mis-named, think of it as post-init.)
+
+    extra - After everything is started.
+

stack.sh

@@ -668,6 +668,18 @@ fi
 # Do the ugly hacks for borken packages and distros
 $TOP_DIR/tools/fixup_stuff.sh
 
+
+# Extras Pre-install
+# ------------------
+
+# Phase: pre-install
+if [[ -d $TOP_DIR/extras.d ]]; then
+    for i in $TOP_DIR/extras.d/*.sh; do
+        [[ -r $i ]] && source $i stack pre-install
+    done
+fi
+
+
 install_rpc_backend
 
 if is_service_enabled $DATABASE_BACKENDS; then