Add server and host columns to data_imports to fix #1278

This adds 'server' and 'host' data to DataImport, so you can display that information without parsing. @matallo, this is what you need, isn't it? Old imports will have that column empty, but a person who is debugging can take a look at the log if he want. 
@rafatower CR this, please. This is the first time I upload a migration to CartoDB, be extra careful with that, please ;-)