hook failed: "install" on clean Bionic installation

[Vladimir Grevtsev]

GPG key of Cassandra APT repo is missing or it is invalid, despite config.yaml says "# Apache and DataStax package signing keys are added automatically."

unit-cassandra-1: 16:14:46 DEBUG unit.cassandra/1.juju-log Adding key lib/apache.key
unit-cassandra-1: 16:14:47 DEBUG worker.uniter.jujuc running hook tool "juju-log"
unit-cassandra-1: 16:14:47 DEBUG unit.cassandra/1.juju-log Adding key lib/datastax.key
unit-cassandra-1: 16:14:48 DEBUG unit.cassandra/1.install Hit:1 http://nova.clouds.archive.ubuntu.com/ubuntu bionic InRelease
unit-cassandra-1: 16:14:48 DEBUG unit.cassandra/1.install Hit:2 http://security.ubuntu.com/ubuntu bionic-security InRelease
unit-cassandra-1: 16:14:48 DEBUG unit.cassandra/1.install Hit:3 http://nova.clouds.archive.ubuntu.com/ubuntu bionic-updates InRelease
unit-cassandra-1: 16:14:48 DEBUG unit.cassandra/1.install Hit:5 http://nova.clouds.archive.ubuntu.com/ubuntu bionic-backports InRelease
unit-cassandra-1: 16:14:49 DEBUG unit.cassandra/1.install Get:4 https://dl.bintray.com/apache/cassandra 311x InRelease [3183 B]
unit-cassandra-1: 16:14:49 DEBUG unit.cassandra/1.install Err:4 https://dl.bintray.com/apache/cassandra 311x InRelease
unit-cassandra-1: 16:14:49 DEBUG unit.cassandra/1.install The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E91335D77E3E87CB
unit-cassandra-1: 16:14:52 DEBUG unit.cassandra/1.install Reading package lists...
unit-cassandra-1: 16:14:52 DEBUG unit.cassandra/1.install W: GPG error: https://dl.bintray.com/apache/cassandra 311x InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E91335D77E3E87CB
unit-cassandra-1: 16:14:52 DEBUG unit.cassandra/1.install E: The repository 'http://www.apache.org/dist/cassandra/debian 311x InRelease' is not signed.
unit-cassandra-1: 16:14:52 DEBUG worker.uniter.jujuc running hook tool "juju-log"
unit-cassandra-1: 16:14:52 INFO unit.cassandra/1.juju-log Failed executing 'add-apt-repository --yes deb http://www.apache.org/dist/cassandra/debian 311x main'. Will retry in 10 seconds

[Vladimir Grevtsev (vlgrevtsev)]

Same for cqlsh charm.

Workaround: execute an "sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E91335D77E3E87CB" before charm will put itself into the errored state.