Allow to override enabled parameter in auth block

[Igorshp]

The current template forces enabled flag to always be True if the block is present.
While for most additional providers this works, however it makes it difficult to disable the basic provider as it's enabled by default. (for example when using auth proxy instead).

This PR allows passing enabled parameter to the auth sections. If missing, it will be set to True by default (for backwards compatibility with current config)

Example config:

 vars:
     grafana_auth:
       basic:
         enabled: false
       proxy:
         header_name: X-Forwarded-Email
         header_property: email
         auto_sign_up: true

[Igorshp]

This is interesting. The tests are failing because of test config.
It's asking for

grafana_auth:
  basic: true

ERROR:  argument of type 'bool' is not iterable

It would make sense to keep uniformity and change documentation for basic block to look like:

grafana_auth:
  basic:
    enabled: True

Would love to hear some thoughts on this.

[lock]

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