ipatests: fix test_full_backup_and_restore
The test is failing when calling (on the replica)
ipa-replica-manage re-initialize --from <master>
because the tool needs to resolve master.
The test does not set /etc/resolv.conf on the replica, as a
consequence it relies on whatever DNS server is configured in
your test environment prior to launching the test, and makes
the test unreliable.
In PR-CI env, /etc/resolv.conf points to the machine hosting
the replica vm, which is unable to resolve master.ipa.test.
The fix is modifying the replica's /etc/resolv.conf to use the
master as DNS.
Fixes https://pagure.io/freeipa/issue/7778
Reviewed-By: Christian Heimes <cheimes@redhat.com>