Make the flags configurable using environment variables

[bzon]

This PR makes the flags configurable using environment variables using the ff package:

Usage of clickhouse-exporter:
  -insecure
    	Ignore server certificate if using https. Override environment (CLICKHOUSE_INSECURE) (default true)
  -password string
    	Clickhouse password. Override environment (CLICKHOUSE_PASSWORD)
  -scrape_uri string
    	URI to clickhouse http endpoint. Override environment (CLICKHOUSE_SCRAPE_URI) (default "http://localhost:8123/")
  -telemetry.address string
    	Address on which to expose metrics. Override environment (CLICKHOUSE_TELEMETRY_ADDRESS) (default ":9116")
  -telemetry.endpoint string
    	Path under which to expose metrics. Override environment (CLICKHOUSE_TELEMETRY_ENDPOINT) (default "/metrics")
  -user string
    	Clickhouse user. Override environment (CLICKHOUSE_USER)

The reason for this is we want to make the scrape_uri configurable using environment variables.

[maxianwei]

good

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