selftest: Avoid system krb5.conf in "none" test env

selftest: Avoid system krb5.conf in "none" test env

Some torture tests do not perform Kerberos activity and do not
run against a server (hence the "none" test env), but do create
a krb5 context, and that causes the Kerberos libs to read
krb5.conf and choke if they don't understand it.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11576

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit f9d6be3b749313a03e9097d848ce078f19197a0a)

Autobuild-User(v4-3-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-3-test): Wed Oct 28 14:01:49 CET 2015 on sn-devel-104