Add multiple cluster networks support to tests

Add possibility to deploy environments with two sets of
L2 networks in the system tests. This patch implements
support of defining cluster nodes groups and editing of
networks settings based on network group name. Also, due
to lack of automatic cobbler configuring add the method
which allows to rewrite default dnsmasq template.

Change-Id: Ib59b5cc1073b4c44c5d142ab20fe9feebf56d3ee
Implements: blueprint multiple-l2-networks-test
(cherry picked from commit 150f56916d797ce5582916712aa8e930e6538a39)
Merge "Add multiple cluster networks support to tests" into stable/6.0