[install bug]Fix directory changed when installing env
[install bug]Fix directory changed when installing env

The current directory is changed when install_rally.sh installs
virtual environment, so if we use ./install_rally.sh, $BASEDIR
will be an error directory when we decide whether downloading rally.

Change-Id: I5939c9e1cb93209057c34c9617d0c03512ebb52f

openstack/rally,yeming233/rally,openstack/rally,yeming233/rally,openstack/rally,openstack/rally