Provide script functionality for creating update mirrors

Update mirror generates differential mirrors between versions
 We have several ways for creating mirrors:
 1) simple comparation of two mirror folders (the easyest way)
   Should be run on server (or client) where we have two
   ready mirrors (for example for ver. 5.1 and 6.0)
   6.0 - 5.1 = update_mirror
 2) more complex implementation (good for make process)
   in this case we build PRODUCT_VERSION mirror and then
   download and process the difference in packages
   between PRODUCT_VERSION..master (Works good for RPM,
   not so good fo DEB).

Implements-blueprint: generating-update-mirrors
Change-Id: Ic208b28e87a1505fe12317494ad327841c9a0964