[17.08][ocata][pike] mongodb (shared-db) relation is required unconditionally

With gnocchi used for measure storage and ceilometer API deprecation we no longer need mongodb to be always present. However, we still need to maintain compatibility with old OpenStack versions and telemetry stack where ceilometer has metric storage responsibilities. Another complication is that ceilometer-upgrade in the ceilometer code itself tries to use mongodb on upgrade even if it is not configured.

https://github.com/openstack/charm-ceilometer/commit/72522a34

The problematic code-path:

assess_status|
https://github.com/openstack/charm-ceilometer/blob/stable/17.08/lib/ceilometer_utils.py#L355-L369
assess_status_func
https://github.com/openstack/charm-ceilometer/blob/stable/17.08/lib/ceilometer_utils.py#L384-L397

resolve_required_interfaces
required_ints = deepcopy(REQUIRED_INTERFACES)
https://github.com/openstack/charm-ceilometer/blob/stable/17.08/lib/ceilometer_utils.py#L372-L381

REQUIRED_INTERFACES = {
    'database': ['mongodb'],
https://github.com/openstack/charm-ceilometer/blob/stable/17.08/lib/ceilometer_utils.py#L104-L105

James pointed out that mongodb needs to be present on Ocata even if it is not needed:

https://github.com/openstack/charm-ceilometer/commit/72522a34#diff-d9b91c0550e242fbf5d1e4ee86924a35R161

This is due to the fact that the database upgrade code on Ocata

https://github.com/openstack/ceilometer/blob/stable/ocata/releasenotes/notes/rename-ceilometer-dbsync-eb7a1fa503085528.yaml

https://github.com/openstack/ceilometer/blob/stable/ocata/setup.cfg#L265
https://github.com/openstack/ceilometer/blob/stable/ocata/ceilometer/cmd/storage.py#L30-L53

https://github.com/openstack/ceilometer/commit/779673534fefc39633aed18bb020885bd6020e06

requires mongodb to be present unconditionally:

"connection" or "metering_connection" fetched by get_connection_from_config:
https://github.com/openstack/ceilometer/blob/stable/ocata/ceilometer/storage/__init__.py#L71-L73

get_connection_from_config used in "def upgrade":
https://github.com/openstack/ceilometer/blob/stable/ocata/ceilometer/cmd/storage.py#L46

on the charm side, connection option is only rendered if mongodb is present
https://github.com/openstack/charm-ceilometer/blob/stable/17.08/templates/ocata/ceilometer.conf#L43-L53

ceilometer_upgrade executes ceilometer-upgrade which triggers that ceilometer code:
https://github.com/openstack/charm-ceilometer/blob/master/lib/ceilometer_utils.py#L461-L470

--

We need to figure out some way around this while providing compatibility and a clear upgrade experience.

Could try to backport this change to Ocata https://github.com/openstack/ceilometer/commit/7905ff3a3 as well.