Build wheel cache for venvs

Building a bunch of virtual envs later is going to be tedious if we do not
pre-cache certain annoying-to-build packages.

* tools/build_wheels.sh: pre-build some wheels for annoying package installs
* list distro package dependencies in files/*/venv
* list packages to pre-build as wheels in files/venv-requirements.txt
* install database Python modules when setting up the database

Change-Id: Idff1ea69a5ca12ba56098e664dbf6924fe6a2e47
diff --git a/clean.sh b/clean.sh
index 50d414c..fea1230 100755
--- a/clean.sh
+++ b/clean.sh
@@ -119,6 +119,10 @@
     sudo rm -rf $SCREEN_LOGDIR
 fi
 
+# Clean up venvs
+DIRS_TO_CLEAN="$WHEELHOUSE"
+rm -rf $DIRS_TO_CLEAN
+
 # Clean up files
 
 FILES_TO_CLEAN=".localrc.auto docs/files docs/html shocco/ stack-screenrc test*.conf* test.ini*"