Add support for providing additional HTTP headers

[gw0]

Sometimes the Grafana server is behind a proxy or application firewall and it is necessary to provide additional HTTP headers to establish a connection, such as authentication tokens or cookies. In general sense these are represented as key-value pairs of strings. To also add support to provide these using an environment variable GRAFANA_HTTP_HEADERS it should contain a JSON-encoded values.

Usage:

provider "grafana" {
  url  = "http://172.17.0.1:3000"
  auth = "admin:admin"
  headers = {
    "Cookies" = "foo=bar"
  }
}

$ GRAFANA_HTTP_HEADERS='{"Cookies":"foo=bar"}' terraform plan

Depends on PR nytm/go-grafana-api#63.

[gw0]

Due to changed dependency this PR was replaced with PR #134.