Recent virtualenv review cleanups

This is a follow-on to comments in https://review.openstack.org/156356
and https://review.openstack.org/#/c/151513/

* Remove work-around for /var/cache/pip
* Remove WHEELHOUSE setting in tools/build_wheels.sh and use the pip
  default directory '<cwd>/wheelhouse'
* Remove bogus MySQL-python install
* Removed unused bits and clean up pip commands in from tools/build_venvs.sh

Closes-Bug: #1423720
Change-Id: I0283b0dff9146b1b63bd821358505a93566270c6
diff --git a/stack.sh b/stack.sh
index 6002cf9..f597f78 100755
--- a/stack.sh
+++ b/stack.sh
@@ -677,14 +677,8 @@
 # Virtual Environment
 # -------------------
 
-# Temporary hack for testing
-# This belongs in d-g functions.sh setup_host() or devstack-vm-gate.sh
-if [[ -d /var/cache/pip ]]; then
-    sudo chown -R $STACK_USER:$STACK_USER /var/cache/pip
-fi
-
 # Pre-build some problematic wheels
-if [[ ! -d ${WHEELHOUSE:-} ]]; then
+if [[ -n ${WHEELHOUSE:-} && ! -d ${WHEELHOUSE:-} ]]; then
     source $TOP_DIR/tools/build_wheels.sh
 fi