I have exactly the same issue on 16.04:
[172512.094995] audit: type=1400 audit(1482614869.625:1439): apparmor="DENIED" operation="change_onexec" info="no new privs" error=-1 namespace="root//lxd-torelay_<var-lib-lxd>" profile="unconfined" name="system_tor" pid=128522 comm="(tor)" target="system_tor"
I have exactly the same issue on 16.04:
[172512.094995] audit: type=1400 audit(148261486 9.625:1439) : apparmor="DENIED" operation= "change_ onexec" info="no new privs" error=-1 namespace= "root// lxd-torelay_ <var-lib- lxd>" profile= "unconfined" name="system_tor" pid=128522 comm="(tor)" target="system_tor"