Replace "sid" and "testing" by "stretch"
Replace "sid" and "testing" by "stretch"

Supported Debian distros (codenames) are "sid", "testing",
and "jessie", but it should be "stretch" and "jessie".

"testing" is no codename and therefore should be replaced by
"stretch".

"sid" changes all the time and cannot be guaranteed to run
correctly or is at least not tested.

Change-Id: Id4b80a055452bbff69036d4dc1adeda46ce99664
Closes-Bug: #1673810
Closes-Bug: #1674416

nawawi/openstack,dtroyer/devstack,richliu/devstack,group-policy/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack,group-policy/devstack,noironetworks/devstack,richliu/devstack