cni manage_flags fails if no cni relations are present

We use goal-state to determine configuration for related CNI endpoints in interface-kubernetes-cni:

https://github.com/juju-solutions/interface-kubernetes-cni/commit/d5754e3f50a13e35b224842b6dae5dcae989cca0#diff-6e152090b45a29ed86305121942fb300R40

If no cni relations have been specified, we'll get a key error. We don't typically hit this because we deploy with bundles, which set up required relations at deploy time. However, deploying a k8s-[master|worker] charm by itself will fail. Repro:

juju deploy ~containers/kubernetes-master --constraints mem=4G --channel beta --config channel=1.18/beta

When manage_flags runs for the cni relation, we'll hit this:

...
unit-kmb-0: 10:42:35 INFO unit.kmb/0.juju-log Reactive main running for hook install
unit-kmb-0: 10:42:35 ERROR unit.kmb/0.juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-kmb-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
    hookenv._run_atstart()
  File "/var/lib/juju/agents/unit-kmb-0/.venv/lib/python3.6/site-packages/charmhelpers/core/hookenv.py", line 1312, in _run_atstart
    callback(*args, **kwargs)
  File "/var/lib/juju/agents/unit-kmb-0/.venv/lib/python3.6/site-packages/charms/reactive/endpoints.py", line 132, in _startup
    endpoint._manage_flags()
  File "/var/lib/juju/agents/unit-kmb-0/.venv/lib/python3.6/site-packages/charms/reactive/endpoints.py", line 251, in _manage_flags
    self.manage_flags()
  File "/var/lib/juju/agents/unit-kmb-0/charm/hooks/relations/kubernetes-cni/provides.py", line 13, in manage_flags
    self.config_available())
  File "/var/lib/juju/agents/unit-kmb-0/charm/hooks/relations/kubernetes-cni/provides.py", line 31, in config_available
    name for name in goal_state['relations'][self.endpoint_name]
KeyError: 'cni'
...