No retries when pre-install-hook apt-install fails

[Jason Hobbs]

When this line fails, there are no retries performed:

apt-get install -y python python-apt python-yaml

2019-09-28 06:02:26 INFO juju-log Invoking charm-pre-install hooks
2019-09-28 06:02:27 INFO juju-log Ensuring python, python-apt and python-yaml are installed
2019-09-28 06:02:27 DEBUG install E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
2019-09-28 06:02:27 DEBUG install E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
2019-09-28 06:02:27 ERROR juju.worker.uniter.operation runhook.go:132 hook "install" failed: exit status 100

We get a hook error.