testing: Cache packages downloaded with pip for strongTNC

This way no network connections is required to rebuild the root/guest images.

testing/scripts/recipes/015_strongTNC.mk

@@ -3,6 +3,7 @@
 PKG = strongTNC
 ZIP = $(PKG)-master.zip
 SRC = https://github.com/strongswan/$(PKG)/archive/master.zip
+DEPS = $(PKG)-deps
 
 all: install
 
@@ -12,6 +13,10 @@ $(ZIP):
 $(PKG)-master: $(ZIP)
 	unzip -u $(ZIP)
 
-install: $(PKG)-master
-	cd $(PKG)-master && pip install -r requirements.txt
+$(DEPS): $(PKG)-master
+	mkdir -p $(DEPS)
+	pip install --download $(DEPS) -r $(PKG)-master/requirements.txt
+
+install: $(DEPS)
+	pip install --no-index --find-links=file://`pwd`/$(DEPS) -r $(PKG)-master/requirements.txt
 	cp -r $(PKG)-master /var/www/tnc && chgrp -R www-data /var/www/tnc