Update the git submodules and fix fallout:
* Add ops-interface-tls-certificates back in to deps update script
* Charmhelpers has been switched to using pbr which broke the
update-deps target. The problem seems to be that pbr cannot detect
the version of charm-helpers when using a submodule and deploying to
a local dir. To workaround this set PBR_VERSION manually.
* cluster cannot be set as a extra binding anymore ("ERROR cannot
deploy bundle: charm "ceph-iscsi" has invalid extra bindings:
relation names (cluster) cannot be used in extra bindings")
* Charm class should no longer provde a `key` argument in __init__ *1
* framework.observe must now pass the method to be invoked as
the last argument *2
*1 https://github.com/canonical/operator/blob/master/ops/main.py#L309
*2 https://github.com/canonical/operator/blob/master/ops/framework.py#L553
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