Add an update option to run_tests.sh
Add an update option to run_tests.sh

This commit adds an update option to run_tests.sh. This option
is used to rerun install_venv.py on an already installed venv.
This will then just update out of date packages with pip.

Change-Id: I96dc6940f536feb810d57b3316e9d712aa750d93

rickerc/cinder_audit,petrutlucian94/cinder,redhat-openstack/cinder,tlakshman26/cinder-new-branch,tlakshman26/cinder-new-branch,blueboxgroup/cinder,NetApp/cinder,j-griffith/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,alex8866/cinder,julianwang/cinder,Thingee/cinder,hguemar/cinder,sasukeh/cinder,hguemar/cinder,redhat-openstack/cinder,duhzecca/cinder,winndows/cinder,nikesh-mahalka/cinder,maelnor/cinder,Hybrid-Cloud/cinder,mahak/cinder,tlakshman26/cinder-https-changes,Akrog/cinder,nikesh-mahalka/cinder,JioCloud/cinder,apporc/cinder,scottdangelo/RemoveVolumeMangerLocks,spring-week-topos/cinder-week,Accelerite/cinder,Thingee/cinder,potsmaster/cinder,Paul-Ezell/cinder-1,rickerc/cinder_audit,CloudServer/cinder,rakeshmi/cinder,github-borat/cinder,phenoxim/cinder,duhzecca/cinder,Nexenta/cinder,j-griffith/cinder,dims/cinder,saeki-masaki/cinder,manojhirway/ExistingImagesOnNFS,NeCTAR-RC/cinder,maelnor/cinder,scality/cinder,petrutlucian94/cinder,eharney/cinder,saeki-masaki/cinder,spring-week-topos/cinder-week,Thingee/cinder,Datera/cinder,tlakshman26/cinder-https-changes,github-borat/cinder,apporc/cinder,cloudbau/cinder,julianwang/cinder,nexusriot/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,cloudbase/cinder,NeCTAR-RC/cinder,bswartz/cinder,mahak/cinder,nexusriot/cinder,manojhirway/ExistingImagesOnNFS,dims/cinder,bswartz/cinder,Nexenta/cinder,inkerra/cinder,potsmaster/cinder,sasukeh/cinder,scality/cinder,inkerra/cinder,eharney/cinder,Hybrid-Cloud/cinder,scottdangelo/RemoveVolumeMangerLocks,takeshineshiro/cinder,JioCloud/cinder,leilihh/cinder,alex8866/cinder,phenoxim/cinder,abusse/cinder,cloudbau/cinder,Akrog/cinder,ge0rgi/cinder,openstack/cinder,ntt-sic/cinder,NetApp/cinder,takeshineshiro/cinder,winndows/cinder,Datera/cinder,openstack/cinder,Paul-Ezell/cinder-1,cloudbase/cinder,Accelerite/cinder,ntt-sic/cinder,leilihh/cinder,CloudServer/cinder,abusse/cinder,blueboxgroup/cinder,rakeshmi/cinder