cli: allow the value for --as-user to be specified as env var

* as fallback, the parameter --as-user/-u now reads its value from the
  environment variable INVENIO_UTILITIES_TUW_USER (if it is set)
* thus, the --as-user/-u parameter doesn't have to be specified for each
  command anymore
* the value is still required, s.t. either the env var or the CLI option
  have to be specified