Missing check on manage_lbs causes charm to error if lb relation comes before juju trust command

[Pedro Guimarães]

When deploying and sharing credentials with juju trust, I've noticed that we have no checks for manage_lbs() method as we have for handle_requests().

That may cause the following error at the begining of the deployment:
2021-07-22 16:40:24 ERROR juju-log lb-consumers:1: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-azure-integrator-0/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-azure-integrator-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-azure-integrator-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-azure-integrator-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-azure-integrator-0/charm/reactive/azure.py", line 100, in manage_lbs
    _create_lb(request)
  File "/var/lib/juju/agents/unit-azure-integrator-0/charm/reactive/azure.py", line 112, in _create_lb
    request.response.address = layer.azure.create_loadbalancer(request)
  File "/var/lib/juju/agents/unit-azure-integrator-0/charm/lib/charms/layer/azure.py", line 325, in create_loadbalancer
    _azure("network", *lb_create_args)
  File "/var/lib/juju/agents/unit-azure-integrator-0/charm/lib/charms/layer/azure.py", line 733, in _azure
    raise AzureError.get(stderr)
charms.layer.azure.AzureError: ERROR: Please run 'az login' to setup account.