fix: [python-venv] python version was hardwritten and couldn't be changed without editing the charm.

Now this is a valid charm variable that can be overridden via charm
config files or on command line call.

precise/python-venv/hooks/install

@@ -2,7 +2,7 @@
 
 set -eux # -x for verbose logging to juju debug-log
 
-VERSION="2.7.5"
+VERSION=${VERSION:-"2.7.9"}
 
 ##
 ## INSTALL Python