run_tests.sh: make integrated test to run with virtual env

Otherwise
> ...
> Running integrated test ...
> sudo: nosetests: command not found
> Build step 'Execute shell' marked build as failure
> Sending e-mails to: ntt-nos-jenkins@lists.valinux.co.jp
> Finished: FAILURE

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>