Data Loader

[DEV-4271] added possibility to define item key for item generation

Merge in ZT/data-loader from feature/DEV-4271 to master

* commit 'c128f835a929db7f0b707f729478c189f13d3a67':
  [DEV-4271] added %1$s as backward compatible item key generation
  [DEV-4271] updated README item generation section
  [DEV-4271] added support of item_key for host item generator