buildserver: support HTTPS Debian mirrors

The ever troublesome gpjenkins box needs to use HTTPS mirrors.  Plus it
improves the security of the buildserver, since there have been CVEs that
HTTPS would protect against:
https://www.debian.org/security/2016/dsa-3733

buildserver/provision-apt-get-install

@@ -6,14 +6,19 @@ set -x
 
 debian_mirror=$1
 
-sed -i "s,http://ftp.uk.debian.org/debian/,${debian_mirror},g" /etc/apt/sources.list
-
 printf 'APT::Install-Recommends "0";\nAPT::Install-Suggests "0";\n' \
        > /etc/apt/apt.conf.d/99no-install-recommends
 
 printf 'APT::Acquire::Retries "20";\n' \
        > /etc/apt/apt.conf.d/99acquire-retries
 
+if echo $debian_mirror | grep '^https' 2>&1 > /dev/null; then
+    apt-get -y update
+    apt-get -y install apt-transport-https
+fi
+
+sed -i "s,http://ftp.uk.debian.org/debian/,${debian_mirror},g" /etc/apt/sources.list
+
 if grep --quiet jessie /etc/apt/sources.list; then
     echo "deb $debian_mirror jessie-backports main" > /etc/apt/sources.list.d/backports.list
     echo "deb $debian_mirror testing main" > /etc/apt/sources.list.d/testing.list

jenkins-build-makebuildserver

@@ -46,7 +46,7 @@ export VAGRANT_HOME=$WORKSPACE/vagrant.d
 mkdir $VAGRANT_HOME
 
 cd $WORKSPACE
-echo "debian_mirror = 'http://ftp.uk.debian.org/debian/'" > $WORKSPACE/makebuildserver.config.py
+echo "debian_mirror = 'https://deb.debian.org/debian/'" > $WORKSPACE/makebuildserver.config.py
 echo "boot_timeout = 1200" >> $WORKSPACE/makebuildserver.config.py
 echo "apt_package_cache = True" >> $WORKSPACE/makebuildserver.config.py
 ./makebuildserver --verbose --clean