feat: set log level with env variable

[inspirit941]

hello. I'm currently using python knative functions with this template very well.

When I create python knative functions and try to connect it with some http webhook services that does not support cloudevent spec, it seems that I cannot handle a log comming from here;

parliament/parliament/server.py

Line 33 in 0ec7dba

app.logger.warning('No CloudEvent available')

so I thought it would be convenient if we can handle Flask log level by environment variable provided in knative functions.

example usage when creating knative function (func.yaml)

specVersion: 0.0.0
...
...
envs:
- name: LOG_LEVEL
  value: ERROR
annotations: {}
options: {}
labels: []
healthEndpoints:
  liveness: /health/liveness
  readiness: /health/readiness
created: 
invocation:
  format: http

[sferich888]

@inspirit941 I am trying to address something similar with #26 by simply exposing why the failures are happening.

[matejvasek]

/approve
/lgtm