Pass GENERATE_HASHES to the tox test environment
Pass GENERATE_HASHES to the tox test environment

When bumping nova object versions, it's necessary to generate new
hashes for those that have changed. New hashes are generated when the
GENERATE_HASHES environment variable is set. Let tox copy GENERATE_HASHES
to the test environment to allow object hash generation for a command
line like:

 GENERATE_HASHES=1 tox -epy27

Change-Id: Id652a7a9fbaeaf35db06744cbaf2484504fc2531

klmitch/nova,vmturbo/nova,hanlind/nova,gooddata/openstack-nova,Juniper/nova,klmitch/nova,rajalokan/nova,cloudbase/nova,Juniper/nova,openstack/nova,sebrandon1/nova,cloudbase/nova,klmitch/nova,jianghuaw/nova,sebrandon1/nova,vmturbo/nova,klmitch/nova,cloudbase/nova,mahak/nova,jianghuaw/nova,hanlind/nova,rajalokan/nova,vmturbo/nova,sebrandon1/nova,openstack/nova,jianghuaw/nova,mahak/nova,Juniper/nova,jianghuaw/nova,phenoxim/nova,mikalstill/nova,hanlind/nova,mahak/nova,gooddata/openstack-nova,vmturbo/nova,rajalokan/nova,rahulunair/nova,gooddata/openstack-nova,gooddata/openstack-nova,mikalstill/nova,rahulunair/nova,alaski/nova,rahulunair/nova,mikalstill/nova,openstack/nova,rajalokan/nova,alaski/nova,Juniper/nova,phenoxim/nova