extend character validations for model,fields,tags and measurement names , fix #392

[toni-moreno]

Hi @regel here the PR , I've removed dotted_key as in standar key the dot is included. Aso added test key as suggested last week.
Currently tested and working with this image tonimoreno/loudml:1.6.2

as you can see here.

> show-model MY_MODEL
- settings:
    bucket_interval: 10m
    default_bucket: XXXXXXX
    features:
    - anomaly_type: low_high
      field: value
      io: io
      match_all:
      - tag: metric_id
        value: '2123123'
      measurement: traffic.rxt_et-1/0/0 <---------------------------HERE OK!
      metric: mean
      name: mean_value
    grace_period: 0
    interval: 60s
    max_evals: 20
    max_threshold: 70
    min_threshold: 50
    name: MY_MODEL
    offset: 1h
    run:
      flag_abnormal_data: true
      output_bucket: XXXXXXXXXXXXXXXXXX
      save_output_data: true
    seasonality:
      daytime: true
      weekday: true
    span: 256
    type: donut

[toni-moreno]

Hello @regel , I would like to know about the status of this PR. it seems ready to merge