Don't overwrite existhing venv dir when using --yes

The --yes flag sets OVERWRITEDIR yes currently. Then the user will lost
his data who uses this flag and doesn't know how many yes/no questions
are there. This patch removes the default value of OVERWRITEDIR when
using --yes.

Actually we could set default yes for adding something, but not for
deleting something or security related questions. An example is that yum
still asks you whether add a GPG key when installing packages although
you have already used --yes.

Change-Id: I9341b6b2eb8e33db379e3a2bad94c5d86287a518
Partial-Bug: #1470381

install_rally.sh

@@ -506,7 +506,6 @@ do
             ;;
         -y|--yes)
             ASKCONFIRMATION=0
-            OVERWRITEDIR=yes
             ;;
         --url)
             shift