Upgrade failed with the following:
Installing Munin (system monitoring)…
FAILED: apt-get -y -o Dpkg::Options::=–force-confdef -o Dpkg::Options::=–force-confnew install munin munin-node libcgi-fast-perl
E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?