Improves the logic around ppa addition

kickstart will check if the ppa has already been added and only update
the package cache the first time it is added. Further updates must be
handled by hand.

It also installs the required packages to manage the ppa over command
line if it is not present yet.

kickstart/recipes/kickstart/apt.sh

@@ -1,6 +1,13 @@
 kickstart.apt.ppa() {
+  if ! cat /etc/apt/sources.list.d/* | kickstart.stream.contains "${1#ppa:}"; then
+    if ! kickstart.command_exists add-apt-repository; then
+      kickstart.info "Required add-apt-repository command not found. Installing package."
+      kickstart.package.install python-software-properties || kickstart.package.install software-properties-common
+    fi
+
     kickstart.mute add-apt-repository -y "$1"
     kickstart.package.update
+  fi
 }
 
 kickstart.apt.add_key_from_url() {