Verify if PPA is available for current distribution

Added checking to verify if current distribution (release name, ex. focal) has available PPA packages. If not, packages will be installed from Ubuntu Package archive.

Solves Issue https://github.com/arakasi72/rtinst/issues/518 and https://github.com/arakasi72/rtinst/issues/522.

scripts/rtletsencrypt

@@ -11,6 +11,8 @@
 
 headless=0
 osname=$(lsb_release -si)
+releasename=$(lsb_release -cs)
+
 [ -z $logfile ] && logfile="/dev/null"
 if [ -z $serverip ]; then
   headless=1
@@ -59,11 +61,15 @@ if [ $(dpkg-query -W -f='${Status}' "certbot" 2>/dev/null | grep -c "ok installe
   echo "Installing certbot"
 
   if [ "$osname" = "Ubuntu" ]; then
-    apt-get -q update >> $logfile 2>&1
-    add-apt-repository -y universe >> $logfile 2>&1
-    add-apt-repository -y ppa:certbot/certbot >> $logfile 2>&1
-    apt-get -q update >> $logfile 2>&1
-    apt-get install -y certbot python-certbot-nginx >> $logfile 2>&1
+    # Verify if current distribution (release name, ex. focal) has available PPA packages
+    wget -qO- http://ppa.launchpad.net/certbot/certbot/ubuntu/dists/ | grep -q "$releasename" && USE_PPA=y
+    if [ "$USE_PPA" = "y" ]; then
+      apt-get -q update >> $logfile 2>&1
+      add-apt-repository -y universe >> $logfile 2>&1
+      add-apt-repository -y ppa:certbot/certbot >> $logfile 2>&1
+    fi
+      apt-get -q update >> $logfile 2>&1
+      apt-get install -y certbot python-certbot-nginx >> $logfile 2>&1
   else
     deb_name=$(lsb_release --codename --short)
     grep "^deb http\:\/\/deb\.debian\.org\/debian $deb_name-backports main" /etc/apt/sources.list > /dev/null || echo "deb http://deb.debian.org/debian $deb_name-backports main" >> /etc/apt/sources.list