Allow defining a topology with multiple domains

[abbra]

ipatests allow to have multiple test domains with multiple hosts in each. However, ipa-test-config.yml generated by PR-CI code always uses a single IPA domain. All hosts known from ansible inventory are put into that domain with fixed DNS domain name.

For tests against Samba AD and later for IPA-IPA trust I need at least two domains created. Right now, even if I would add a vagrant topology that has four machines, they will still be in the same ipa.test domain. I need the split between machines in domains to be configurable.

Note that this is different from having 'AD' domain. AD domain provisions Windows while here I want to have at least two domains with all machines provisioning Linux.