TT#16679 Enable stretch-security mirror in sources.list, now being available

The mirror:

  deb https://debian-security.sipwise.com/debian-security/ stretch-security main contrib non-free

is available nowadays.

Noticed during review of debian-debug situation.

Change-Id: I7497744afe2430c6848c2b8b39e68bcfba009dc2

deployment.sh

@@ -1350,14 +1350,7 @@ cat > /etc/debootstrap/etc/apt/sources.list << EOF
 deb ${MIRROR} ${DEBIAN_RELEASE} main contrib non-free
 EOF
 
-# drop this once Debian/stretch has security support (AKA released as stable)
-if [ "$DEBIAN_RELEASE" = "stretch" ] ; then
-  echo  "Warning: not enabling security repository for $DEBIAN_RELEASE"
-  logit "Warning: not enabling security repository for $DEBIAN_RELEASE"
-else
-  echo "deb ${SEC_MIRROR} ${DEBIAN_RELEASE}-security main contrib non-free" >> /etc/debootstrap/etc/apt/sources.list
-fi
-
+echo "deb ${SEC_MIRROR} ${DEBIAN_RELEASE}-security main contrib non-free" >> /etc/debootstrap/etc/apt/sources.list
 echo "deb ${MIRROR} ${DEBIAN_RELEASE}-updates main contrib non-free" >> /etc/debootstrap/etc/apt/sources.list
 
 if [ -n "$PUPPET" ] ; then