Configure with --enable-dnstap by default in GitLab CI

All platforms but OpenBSD have fstrm and protobuf-c in their repositories and DNSTAP can be tested there. The DNSTAP testing reach can be broadened by making the --enable-dnstap option the default and disabling DNSTAP where necessary or suitable. For the sake of testing in Linux environments where DNSTAP dependencies are not present, --disable-dnstap was added to Ubuntu Xenial job.

fstrm and protobuf-c need to be added to few CI images. https://gitlab.isc.org/isc-projects/images/-/merge_requests/101 needs to be merged first.