Allow overriding CirrOS location during tempest tests

Issue

• When running the overcloud validation, the command will fail downloading the CirrOS if the server is on an isolated network

• The following command is being used:

openstack overcloud validate --overcloud-auth-url $OS_AUTH_URL -- overcloud-admin-password $OS_PASSWORD

• During the process, the script tries to download a CirrOS image from the Internet, but the host where the tempest tests are ran does not have Internet connection.