Add cirros_version for tests, default 0.3.0

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Fix default url for cirros

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Fix CIRROS_IMAGE to use any cirros version as 'cirros'

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Fix wget syntax

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Move cirros_image from prepare to prepare_tempest

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>