Title: #3516: Split TestTrust class and add it to nightly tests
Move some test cases from TestTrust class to other classes, grouping them by the AD hosts
Different topologies can be used to test different sets of cases.
Add test_trust.py::TestTrustWithRootDomain to nightly tests
To pull the PR as Git branch:
git remote add ghfreeipa https://github.com/freeipa/freeipa
git fetch ghfreeipa pull/3516/head:pr3516
git checkout pr3516