Drupal: Search API : Provide default Search API server which configuration can be overridden with variables

my_module.module

<?php

/**
* Implements hook_default_search_api_server().
*/
function MY_MODULE_default_search_api_server() {
  $items = [];

  $host = variable_get('solr_search_api_host', 'localhost');
  $scheme = variable_get('solr_search_api_scheme', 'http');
  $port = variable_get('solr_search_api_port', '8983');
  $path = variable_get('solr_search_api_path', '\\/solr\\/drupal');
  // Determine Solr version automatically.
  $version = variable_get('solr_search_api_solr_version', NULL);
  $class = variable_get('solr_search_api_solr_class', 'search_api_solr_service');
  $excerpt = variable_get('solr_search_api_solr_excerpt', TRUE);
  $retrieve_data = variable_get('solr_search_api_solr_retrieve_data', TRUE);
  $highligh_data = variable_get('solr_search_api_solr_highlight_data', TRUE);

  $items['master_solr_server'] = entity_import('search_api_server', '{
    "name" : "Master Solr server",
    "machine_name" : "master_solr_server",
    "description" : "",
    "class" : "' . $class . '",
    "options" : {
      "clean_ids" : true,
      "site_hash" : true,
      "scheme" : "' . $scheme . '",
      "host" : "' . $host . '",
      "port" : "' . $port . '",
      "path" : "' . $path . '",
      "http_user" : "",
      "http_pass" : "",
      "excerpt" : "' . $excerpt . '",
      "retrieve_data" : "' . $retrieve_data . '",
      "highlight_data" : "' . $highligh_data . '",
      "skip_schema_check" : 0,
      "solr_version" : "' . $version . '",
      "http_method" : "AUTO"
    },
    "enabled" : "1"
 }');

  return $items;
}