added configuration parameters for validation

[alexandernorth]

This is a quick implementation of a way to disable validators based on the data-server.yaml config file to unblock other users. A refactoring of the validation logic is planned.

Adding the following lines to the root level of the data-server.yaml file will disable all validators:

validation:
  disabled-validators:
    - mandatory
    - leafref
    - leafref-min-max-attributes
    - pattern
    - must-statements
    - length
    - range
#    - max-elements

[steiler]

Thanks Alex.

1. I'd rather go for a struct with explicit validator names.

validator:
   disable_validation:
      must_statements: true
      leafref: true

This would then result in all the fields being false by default and only explicitly true fields are true.
2. Also the global var AvailableValidators I would try to avoid. Globals are usually not to be favoured. Same for LoadedConfig. So add a field to datastore.New that injects the config validation struct and add a parameter to the validation func that takes the config struct as a parameter as well.
4. We should maybe in the config create a datastore top level, push the datastores under it and have the validation also rooted there.

[steiler]

lgtm

[hansthienpondt]

LGTM