Add snapshot-list, snapshot-delete and net-list commands

Make more useful help for dos.py

Change-Id: I924a339173caac350bfb4cc5bc982decb2eb30c4
Support of 'bridge' network implemented for libvirt driver

Implementation of 'bridge' network type that allows to connect
VM to existing host bridge interface

Change-Id: I69ebbde21ce2c9e4063bc402e7ab23a359f173bf