charms.openstack should fail gracefully when release_pkg is unset

[Linda Guo]

 Generated a freenas Cinder storage plugin charm for an iscsi backend by:

$cookiecutter https://github.com/openstack-charmers/cinder-storage-backend-template.git

Charm install failed, because in lib/charm/openstack/cinder_freenas.py, in the charm class, didn't set 'release_pkg = version_package'.

charms_openstack should fail gracefully when release_pkg is unset and provide more helpful debug info

----------------------------------------
2020-11-17 03:30:31 WARNING install Traceback (most recent call last):
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/charm/hooks/install", line 22, in <module>
2020-11-17 03:30:31 WARNING install main()
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms/reactive/__init__.py", line 74, in main
2020-11-17 03:30:31 WARNING install bus.dispatch(restricted=restricted_mode)
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 390, in dispatch
2020-11-17 03:30:31 WARNING install _invoke(other_handlers)
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 359, in _invoke
2020-11-17 03:30:31 WARNING install handler.invoke()
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms/reactive/bus.py", line 181, in invoke
2020-11-17 03:30:31 WARNING install self._action(*args)
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/charm/reactive/layer_openstack.py", line 26, in default_install
2020-11-17 03:30:31 WARNING install with charm.provide_charm_instance() as instance:
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms_openstack/charm/core.py", line 96, in __enter__
2020-11-17 03:30:31 WARNING install return BaseOpenStackCharm.singleton
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms_openstack/charm/core.py", line 302, in singleton
2020-11-17 03:30:31 WARNING install release = _release_selector_function()
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms_openstack/charm/defaults.py", line 108, in default_select_release
2020-11-17 03:30:31 WARNING install apt_cache_sufficient=(not singleton.source_config_key))
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charms_openstack/charm/core.py", line 527, in get_os_codename_package
2020-11-17 03:30:31 WARNING install pkg = cache[package]
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charmhelpers/fetch/ubuntu_apt_pkg.py", line 79, in __getitem__
2020-11-17 03:30:31 WARNING install apt_result = self._apt_cache_show([package])[package]
2020-11-17 03:30:31 WARNING install File "/var/lib/juju/agents/unit-cinder-freenas-0/.venv/lib/python3.5/site-packages/charmhelpers/fetch/ubuntu_apt_pkg.py", line 161, in _apt_cache_show
2020-11-17 03:30:31 WARNING install universal_newlines=True)
2020-11-17 03:30:31 WARNING install File "/usr/lib/python3.5/subprocess.py", line 626, in check_output
2020-11-17 03:30:31 WARNING install **kwargs).stdout
2020-11-17 03:30:31 WARNING install File "/usr/lib/python3.5/subprocess.py", line 693, in run
2020-11-17 03:30:31 WARNING install with Popen(*popenargs, **kwargs) as process:
2020-11-17 03:30:31 WARNING install File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
2020-11-17 03:30:31 WARNING install restore_signals, start_new_session)
2020-11-17 03:30:31 WARNING install File "/usr/lib/python3.5/subprocess.py", line 1490, in _execute_child
2020-11-17 03:30:31 WARNING install restore_signals, start_new_session, preexec_fn)
2020-11-17 03:30:31 WARNING install TypeError: Can't convert 'NoneType' object to str implicitly
2020-11-17 03:30:31 ERROR juju.worker.uniter.operation runhook.go:136 hook "install" (via explicit, bespoke hook script) failed: exit status 1
-----------------------------------------------------------------

[Linda Guo (lihuiguo)]

Fix proposal:

charms.openstack raised a RuntimeError when release_pkg is not set