I confirmed that the version of samba in the xenial-proposed repository (version 2:4.3.11+dfsg-0ubuntu0.16.04.16) does allow the server to start with a %U in the include line while the server role is set to active directory domain controller. I don't have access to a trusty install, so I won't be able to confirm that.
I confirmed that the version of samba in the xenial-proposed repository (version 2:4.3.11+ dfsg-0ubuntu0. 16.04.16) does allow the server to start with a %U in the include line while the server role is set to active directory domain controller. I don't have access to a trusty install, so I won't be able to confirm that.