Rename references to concerto

[pcantera]

Along the project, the 'concerto' word is being used in order to name configuration files, environment variables, used in API headers, ... due a legacy context and be continued with pre-existing style..

Now it's time, as a need, to apply required changes in those contexts - coordinating with IMCO team-; focused on those which affect to integration layers or customer use case.

• FILES:
  brownfield/register.go
  polling/register.go
  README.md
  main.go
  config.go
  webservice.go
  linux_settings.go

• CONFIG FILE
  .concerto
  xml <concerto ...>
  log

• ENVIRONMENT VARIABLES
  CONCERTO_CA_CERT
  CONCERTO_CLIENT_CERT
  CONCERTO_CLIENT_KEY
  CONCERTO_CONFIG
  CONCERTO_ENDPOINT
  CONCERTO_URL
  CONCERTO_BROWNFIELD_TOKEN
  CONCERTO_COMMAND_POLLING_TOKEN
  CONCERTO_SERVER_ID
  CONCERTO_FORMATTER

• Post
  X-Concerto-Brownfield-Token

• concerto-setup
  Tmp file

NOTE: This is not an issue focused on general review for this word -there's another issue and branch focused on this and a general deep review of the project on the way- , but for integration point of view, such as the detected ones.

Expected Behavior

Current Behavior

Possible Solution

Steps to Reproduce (for bugs)

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Context and environment