ipatests: test_trust: Add test to cover lookup of trusdomains

Adds an integration tests that checks that all trustdomains are
able to be found by trustdomain-find command right after the
trust has been established.

Also moves some code to allow easier adding common test cases for
both POSIX and non-POSIX test classes.

https://fedorahosted.org/freeipa/ticket/4208
ipatests: tasks: Fix dns configuration for trusts

Properly configure forwarders to the AD zone with respect to
newly created ipa dnsforwardzone commands.

https://fedorahosted.org/freeipa/ticket/4401
trusts: Validate missing trust secret properly

Detect the situation if the user passes empty trust secret and
error out properly.

https://fedorahosted.org/freeipa/ticket/4266
Merge branch '4208_trustdomain_test' into test
Merge branch '4401_broken_ad_tests' into test
Merge branch '4266_trust_secret_internal_error' into test