Nginx variables support in tnt_* directives

[ja1cap]

Please add ngx variables support support in directives:

• tnt_insert
• tnt_replace
• tnt_delete
• tnt_select
• tnt_update
• tnt_upsert

Example

set $app_space_id 512;
set $app_space_index_id 0;
set $app_select_offset 0;
set $app_select_limit 100;
set $app_select_it "eq"';
set $app_select_format "key=%s&it=%%it";
location /select {
  tnt_pure_result on;
  tnt_select $app_space_id $app_space_index_id $app_select_offset $app_select_limit $app_select_it $app_select_format;
  tnt_pass tnt;
}