Set the search server used by a given index.
drush search-api-set-index-server
index_id : The machine name of an index
server_id : The machine name of a server
Set the default_node_index index to used the my_solr_server server.
drush search-api-set-index-server default_node_index my_solr_server
Alias to set the default_node_index index to used the my_solr_server server.
drush sapi-sis default_node_index my_solr_server