core: reset state if dnet_idc_update_backend has been failed.
pytests: made server node isolated - now server nodes do not know about each other.
pytests: added test case for checking correct handling situation when 2 backends from different nodes has equal group and ids.
tests: created config struct for create_nodes method and moved all argument to it.
Merge pull request #557 from shaitan/master

Reset state if dnet_idc_update_backend has been failed.