Implement devstack external plugins

This is an initial pass at plugin infrastructure for devstack which
allows specifying an external repository via:

enable_plugin <name> <giturl> [branch]

It implements the devstack specification for this at
I173dee3d57967b1d2ffd30e4868a2832aeac97ce

Change-Id: I8e4175313b3cf0b12e981122358b1288a7eb0746
diff --git a/stack.sh b/stack.sh
index 605d3cc..d4f2afc 100755
--- a/stack.sh
+++ b/stack.sh
@@ -564,15 +564,14 @@
 source $TOP_DIR/lib/ldap
 source $TOP_DIR/lib/dstat
 
+# Clone all external plugins
+fetch_plugins
+
 # Extras Source
 # --------------
 
 # Phase: source
-if [[ -d $TOP_DIR/extras.d ]]; then
-    for i in $TOP_DIR/extras.d/*.sh; do
-        [[ -r $i ]] && source $i source
-    done
-fi
+run_phase source
 
 # Interactive Configuration
 # -------------------------
@@ -714,12 +713,7 @@
 # ------------------
 
 # 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
-
+run_phase stack pre-install
 
 install_rpc_backend
 
@@ -865,11 +859,7 @@
 # --------------
 
 # Phase: install
-if [[ -d $TOP_DIR/extras.d ]]; then
-    for i in $TOP_DIR/extras.d/*.sh; do
-        [[ -r $i ]] && source $i stack install
-    done
-fi
+run_phase stack install
 
 if [[ $TRACK_DEPENDS = True ]]; then
     $DEST/.venv/bin/pip freeze > $DEST/requires-post-pip
@@ -1142,11 +1132,7 @@
 # ====================
 
 # Phase: post-config
-if [[ -d $TOP_DIR/extras.d ]]; then
-    for i in $TOP_DIR/extras.d/*.sh; do
-        [[ -r $i ]] && source $i stack post-config
-    done
-fi
+run_phase stack post-config
 
 
 # Local Configuration
@@ -1328,11 +1314,7 @@
 # ==========
 
 # Phase: extra
-if [[ -d $TOP_DIR/extras.d ]]; then
-    for i in $TOP_DIR/extras.d/*.sh; do
-        [[ -r $i ]] && source $i stack extra
-    done
-fi
+run_phase stack extra
 
 # Local Configuration
 # ===================